Pumps – Including means to prevent foreign material settling from...
Patent
1990-11-26
1993-06-22
Bertsch, Richard A.
Pumps
Including means to prevent foreign material settling from...
92 86, 92158, 137565, 137613, 239708, F04B 2104
Patent
active
052211940
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to electrostatic spray coating, and, more particularly, to an apparatus for electrostatically isolating a source of supply of conductive coating materials from electrostatic coating dispensers, and for pumping such coating materials between the source and dispensers.
BACKGROUND OF THE INVENTION
The application of coating materials using electrostatic spraying techniques has been practiced in industry for many years. In these applications, the coating material is discharged in atomized form, and an electrostatic charge is imparted to the atomized particles which are then directed toward a substrate maintained at a different potential to establish an electrostatic attraction for the charged atomized particles. In the past, coating materials of the solvent-based variety, such as varnishes, lacquers, enamels, and the like, were the primary materials employed in electrostatic coating applications. The problem with such coating materials is that they create an atmosphere which is both explosive and toxic. The explosive nature of the environment presents a safety hazard should a spark inadvertently be generated, such as by accidentally grounding the nozzle of the spray gun, which can ignite the solvent in the atmosphere causing an explosion. The toxic nature of the workplace atmosphere created by solvent coating materials can be a health hazard should an employee inhale solvent vapors.
As a result of the problems with solvent-based coatings, the recent trend has been to switch to water-based coatings which reduce the problems of explosiveness and toxicity. Unfortunately, the switch from electrostatically spraying solvent-based coatings to those of the water-based type has sharply increased the risk of electrical shock, which risk was relatively minor with solvent-based coatings. The risk of electrical shock is occasioned in the use of water-based coatings due to their extreme electrical conductivity, with resistivities of such water-based coatings often falling within the range of 100 to 10,000 ohm centimeters. This is in contrast to resistivities of 200,000 to 100,000,000 ohm centimeters for moderately electrically conductive coatings such as metallic paint, and resistivities exceeding 100,000,000 ohm centimeters for solvent-based lacquers, varnishes, enamels and the like.
The relative resistivity of the coating material is critical to the potential electrical shock which may arise during an electrostatic coating operation. With coating materials which are either not electrically conductive or only moderately electrically conductive, the column of coating material which extends from the charging electrode at the tip of the coating dispenser through the hose leading back to the supply tank has sufficient electrical resistance to prevent any significant electrostatic charging of the material in the supply tank or the tank itself. However, when coating material is highly electrically conductive, as are water-based coatings, the resistance of the coating column in the supply hose is very low. As a result, a high voltage charging electrode located in the vicinity of the nozzle of the coating dispenser electrostatically charges not only the coating particles, but the coating material in the hose, the coating material in the supply tank and the supply tank itself. Under these circumstances, operating personnel inadvertently coming into contact with an exposed supply tank or a charged hose or any other charged part of the system risk serious electrical shock unless such equipment is grounded to draw off the electricity. If the equipment is indeed grounded at any point, however, the electrostatics will not function because the high voltage charge would be conducted away from the coating dispenser electrode as well.
One of the methods for reducing the electrical shock problem is disclosed, for example, in U.S. Pat. No. 3,971,337 to Hastings which is owned by the same assignee as this invention. The Hastings patent discloses an apparatus for electrostatically isolating the
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Coeling Kenneth J.
Hartle Ronald J.
Konieczynski Ronald D.
Bertsch Richard A.
Korytnyk Peter
Nordson Corporation
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