Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Patent
1995-06-07
1997-08-05
Beck, Shrive
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
427185, 427189, 427475, 118627, 118699, 118702, 118DIG5, B05D 106, B05B 714, B05C 1110, B05C 1302
Patent
active
056540426
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to an improved method and apparatus for the gas transport of powder, and more particularly, to methods and apparatus which assure delivery of uniform and consistent quantities of powder per unit time to a spray gun for application to the surface of an article to be coated.
BACKGROUND OF THE INVENTION
In a typical powder coating apparatus, powder is maintained in a fluidized state within a hopper having a fluidized bed, and is transported from the hopper to a pump chamber of a powder pump, whereupon an ejector nozzle in the pump directs a high pressure gas stream toward an outlet of the chamber and along an exit tube connected to the chamber. As a result, the powder is conveyed through the exit tube to a spray gun which sprays the powder toward the surface of an article to be coated. The ejector nozzle must be operated at sufficiently high pressure to transport the particles to the end of the exit tube and through the spray gun. During this pumping operation, the high pressure oft he ejector nozzle creates a relatively low pressure at the intake to the pump chamber, thereby drawing powder particles from the powder container and into the chamber for ejection therefrom along the exit tube.
One typical application for an apparatus of this type involves coating the surface of an aluminum joint with powdered solder flux prior to steps of heating and melting the solder flux to weld the connecting aluminum elements and form the joint. The reliability of the connection depends upon the uniformity of the flux coating.
In these and other applications wherein powder particles are gas-transported to coat a surface, including applications which involve electrostatic charging of the particles, problems may result due to adherence and accumulation of the powder particles at the inlet of the pump from the hopper. This occurs with powders that have the property of relatively easy coherence, easy adherence due to viscosity, relatively low slipperiness, or powders which simply do not have good flow characteristics and tend to agglomerate. Powdered solder flux is such a powder. With these types of powders, even powder flow from the hopper to the spray gun is difficult to achieve, and therefore, the amount of powder transported per unit time fluctuates. As a result, it is difficult when spraying these powders to achieve a uniformly thick coating of powder on the article and to consistently apply a uniform coating from one article to the next in a production line situation. This is possibly due to changes in the flow of the powder resulting from a reduction in the cross sectional area at the inlet to the pump or because of the influence of static electricity among the particles due to friction which encourages agglomeration of the powder particles.
It is therefore one objective of the invention to achieve greater uniformity in powder transport per unit time by minimizing or reducing accumulation and adherence of the particles during transport from the powder hopper to the article to be coated.
In other applications for powder coating via gas transport, it is often necessary to intermittently turn the powder pump on and off. One example for the need of this type of operation involves coating articles carried on a conveyor, wherein it is desirable to spray coat the articles at a coating station on the conveyor, and then turn the pump off until the conveyor moves the next article to the coating station. For applications which require ON/OFF operation of a powder coating apparatus, it is desirable to achieve precise control of the powder flow to effectively turn the apparatus off and on at the desired times. Otherwise, powder is wasted. It is also desirable to spray the same quantity of powder on each article.
In the past, pinch valves made of rubber tubes have been used to control intermittent powder flow at a powder hopper. Rubberized pinch valves are pneumatically operated and are relatively simple and inexpensive. However, during closing, these valves have a tendency to close upo
REFERENCES:
patent: 4109027 (1978-08-01), Crose
patent: 4555416 (1985-11-01), Fights et al.
patent: 5173325 (1992-12-01), Knobbe et al.
patent: 5335828 (1994-08-01), Kaiju et al.
patent: 5474609 (1995-12-01), Mulder et al.
patent: 5520735 (1996-05-01), Mulder
Kasahara Shigeki
Masuda Kenichi
Sakurazawa Masashi
Watanabe Takashi
Beck Shrive
Nordson Corporation
Parker Fred J.
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