Fluidized bed powder handling and coating apparatus and methods

Details Fluidized bed powder handling and coating apparatus and methods Fluidized bed powder handling and coating apparatus and methods

1998-05-29

2003-09-16

Beck, Shrive P. (Department: 1762)

Coating apparatus

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

Electrostatic and/or electromagnetic attraction or...

C118SDIG005, C427S459000

Reexamination Certificate

active

06620243

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to powder handling systems and methods and, more specifically, to electrostatic fluidized bed powder coating systems and methods.
BACKGROUND OF THE INVENTION
Powder must be handled and transferred in a wide variety of systems. For example, powder coating technology has generally evolved over several years into several different coating techniques performed with various types of coating systems. With each technique and apparatus, a powder, such as a resinous polymer or paint, is initially adhered to an electrically conductive object or substrate. This initial coating typically involves electrically grounding the object or substrate and electrostatically charging the powder particles such that the electrostatic attraction causes the powder to adhere to the part or substrate in a uniform thickness. This initial powder coating is then cured using heat or other techniques, such as infrared or ultraviolet light, to fully adhere the coating to the part or substrate.
Conventional techniques for adhering the powder particles to an object before curing have included three general types. Two of these coating techniques involve the use of fluidized powder beds. In the first of these techniques, the part is heated and then dipped into a fluidized bed of powder particles. The particles partially coalesce or tackify and thereby stick to the part. The second technique involves electrostatically charging the powder particles such that they emanate in a cloud from a fluidized powder bed. When an electrically conductive, grounded part is placed with this emanating cloud of electrostatically charged powder particles, the charged particles will be attracted to the outer surfaces of the part. The grounded part may be manually placed within a powder coating structure containing the electrostatically charged powder cloud or may be on a conveyor system or automatic feed system which moves parts or the substrate continuously into and out of the coating structure. A third general technique for powder coating also involves electrostatic charging of powder particles, however, this technique utilizes a spray gun. An electrostatic spray gun generally emits powder particles while electrostatically charging those particles by utilizing a charged electrode at the gun nozzle. Guns also exist which emit a stream of powder particles that are charged by a process commonly referred to as tribo charging. In spray coating techniques, the parts to be coated are again electrically grounded and are typically contained within a coating structure or hood having a vacuum system which collects excess sprayed powder. The three general techniques described above may also have many variations.
Each of the above described powder coating techniques has different advantages and limitations. Powder handling in general is also a problematic area in that it often involves a variety of powder filtering, transfer and containment challenges. Powder coating and process equipment generally is connected to powder collection equipment for collecting airborne powder which has not adhered to the part or substrate during the initial coating process. This equipment may also be referred to as powder reclaim equipment and has been free standing structure relative to the powder processing or coating equipment. This results in increased use of floor space and higher associated costs. For example, in a typical electrostatic fluidized bed coating system, excess powder is reclaimed from powder coating enclosure or structure with vacuum applied by a collector including a blower. Within the collector, and upstream of the blower, the powder is the trapped within one or more filters while air exits the collector. Periodically, the filters are internally pulsed with positively pressurized air to disengage the powder from the filter. The powder then may drop into a reclaim hopper located below the powder collector. The reclaimed powder is then transferred manually or by a conveyorized system to the powder processing equipment, such as the powder coating structure or enclosure.
Powder coating equipment, such as described above, also has drawbacks in terms of the ability to adjust the vacuum being applied to the powder coating structure or enclosure. The blower used in the powder collector portion of the system draws a specific amount of air usually designated in cubic feet per minute. One or more conduits may be connected between the blower and the powder coating structure or enclosure and, for adjustment purposes, slide gates have been connected within these conduits to selectively block the air and powder flow. In this way, air and powder being drawn out of the powder coating structure or enclosure may be increased or decreased depending on the position of the slide gate. While some operators have been known to mark the slide gate position at a desired location, this has not been a generally acceptable or precise manner of adjustment. Moreover, the use of a blower assembly in combination with a pulsable filter within a collector is rather cumbersome. In addition, as the filter or filters become clogged with powder, there can be an undesirable change in the level of collection vacuum applied to the powder coating structure or enclosure. This can adversely affect the powder coating process.
To address problems such as these in this general area of powder handling and coating technology, it would be desirable to provide a powder handling and/or processing or coating system which may be automated, compact and more portable, and more easily and precisely adjustable in accordance with the specific application needs.
SUMMARY OF THE INVENTION
The present invention provides powder handling and coating apparatus and methods achieving advantages to address the problems mentioned above as well as other powder coating and handling problems. For example, the invention can provide an automated powder coating system which is relatively compact as compared to prior systems. Powder may be conveniently added to supply a closed loop powder handling system of the invention. The system can also automatically mix reclaimed powder and new or so-called virgin powder prior to conveying the mixture into coating structure associated with the system. Also the system eliminates the need for the primary powder filters typically contained in the powder collection loop and therefore eliminates the change in collection vacuum associated with such filters. Also, powder color and/or powder type may be more easily changed due to the elimination of filters in the powder collection loop and the more compact system configuration.
In fulfillment of these and other advantages, and in accordance with one aspect of the invention, an electrostatic fluidized bed powder coating apparatus is provided which may include typical powder coating structure, powder fluidizing bed structure and an electrostatic charging device disposed to charge the powder such that it emanates from the fluidizing bed. The powder coating structure may be an enclosure which substantially fully encloses a product, part or substrate during a coating operation or may be a structure which has one or more openings to allow automated or manual introduction of such products, parts or substrates. In accordance with this aspect of the invention an enclosed powder accumulator collects excess powder from the powder coating structure during the powder coating operation. In accordance with the invention, a vacuum pump communicates between the powder coating structure and the powder accumulator and is operated by a source of compressed air capable of precise regulation. Due to the use of a vacuum pump in this way, primary powder filters and associated pulse valves are not necessary. This eliminates the significant drawbacks of blower and filter systems as generally used with fluidized bed systems in the past. The vacuum pump precisely controls the negative pressure in the powder coating structure to ensure full, uniform coating. The vacuum pump can also immediately transfer exce

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