Coating apparatus – Projection or spray type
Reexamination Certificate
1997-06-19
2001-12-04
Edwards, Laura (Department: 1734)
Coating apparatus
Projection or spray type
C118SDIG002, C239S296000, C239S297000, C239S298000, C239S300000
Reexamination Certificate
active
06325853
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to applying a liquid to a substrate either as an atomized or nonatomized spray pattern. More particularly, the invention relates to an improved method and apparatus for selectively applying a coating of material on desired regions of a substrate such as a circuit board while avoiding the deposit of coating material on regions to be left uncoated.
BACKGROUND OF THE INVENTION
Typically, circuit boards that require protection from moisture, electric leakage and dust are coated with moisture proof insulative films, known as conformal coatings, such as, acrylic, polyurethane, silicone or epoxy synthetic resins dissolved in a volatile solvent. When applied to clean circuit boards, an insulative resin film of uniform thickness and without pinholes, is formed as the solvent evaporates.
Spraying is the most commonly used insulative coating method employed in mass production. Spraying can be categorized as either air spraying in which an air stream is impinged against the stream of liquid coating material after leaving the spray nozzle to form an atomized spray pattern, or airless spraying in which the coating material is dispensed as a nonatomized spray pattern, as is disclosed in U.S. Pat. No. 5,294,459, assigned to Nordson Corp., which is hereby incorporated by reference in its entirety.
In airless spraying of conformal coating material, as described in the U.S. Pat. No. 5,294,459, the coating material is sprayed onto a circuit board as a flat, nonatomized pattern. Relative movement is effected between the nozzle and the circuit board in a direction transverse to the plane of the flat pattern discharged from the nozzle. The supply of coating material to the nozzle is intermittently interrupted so as to prevent a deposit of liquid coating on regions of the circuit board and/or circuit components which are to be left uncoated.
While air spraying, in which an air stream is impinged against the stream of liquid coating material to atomize the material after leaving the spray nozzle, as disclosed in the U.S. Pat. No. 5,294,459, has proven to be an effective means of applying the material to a substrate, there is still some overspray which increases the production costs of the circuit boards. Even with airless spraying, the distance of the nozzle to the circuit board sometimes causes the coating material to coat regions which are to be left uncoated.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide apparatus and methods for coating substrates, such as circuit boards, with either a bead or an atomized conical, or a swirling spray pattern of liquid coating material discharged from a nozzle mounted to a liquid spray device to obviate the problems and limitations of the prior art systems.
It is a further object of the present invention to provide an apparatus and method having three modes of operation for selectively coating substrates, such as circuit boards, with either a bead of liquid coating material dispensed through an elongated nozzle tip or with an atomized, conical spray pattern formed by directing jets of relatively high pressurized air against the bead of liquid coating material dispensed through the elongated nozzle tip, or with a non-atomized, softswirling spray pattern formed by directing jets of relatively low pressurized air against the bead of liquid coating material dispensed through the nozzle.
It is another object of the present invention to direct a portion of the jets of pressurized air against the bead of liquid coating material dispensed through the elongated nozzle tip and a portion of the pressurized air against the nozzle tip to stabilize the nozzle tip.
It is another object of the present invention to minimize the volume of residual coating material present in the throughbore, consequently minimizing or substantially eliminating drip and dribble during startup.
It is another object of the present invention to minimize pressure spikes when initiating a pressure-assisted mode of operation, consequently minimizing or substantially eliminating splatter during startup.
In accordance with a first embodiment of the invention, a system and method for spray coating a substrate with a liquid coating material includes a liquid spray device having a liquid delivery passageway for supplying liquid coating material and an air delivery passageway for supplying pressurized air. A nozzle mounted to the liquid spray device has an elongated nozzle tip extending outwardly therefrom and a throughbore in flow communication with the liquid delivery passageway. A nozzle orifice disposed at the end of the throughbore discharges the liquid coating material as a bead of liquid coating material. The nozzle includes a plurality of bores in flow communication with the air delivery passage. Each of the bores is formed at an angle with respect to the throughbore to direct pressurized air against both the elongated nozzle tip and the outer periphery of the bead of liquid coating material discharged from the elongated nozzle tip to form the liquid coating material into an atomized, conical swirling spray pattern. A valve controller is connected to the liquid spray device for intermittently opening and closing the air delivery passageway of the liquid spray device whereby the bead of liquid coating material discharged from the nozzle orifice is applied to the substrate either as the atomized, conical, swirling spray pattern or as a bead of the liquid coating material.
In accordance with a second embodiment of the invention, the system and method of the first embodiment can be used with a nozzle constructed without the elongated nozzle tip.
In accordance with a third embodiment of the invention, pressurized air supplied to the spray nozzle is diverted to an exhaust port when operating the spray gun in a non-pressure-assisted mode of operation.
In accordance with a fourth embodiment of the invention, pressurized air supplied to the spray nozzle is reacted by a fluid damping system when initiating operation of the spray gun in a pressure-assisted mode of operation.
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Donges William E.
Fox Kevin
Harrell Greg
Havlik Chris
Hogan Patrick Thomas
Calfee Halter & Griswold LLP
Edwards Laura
Nordson Corporation
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