Patent Application
20070145167
This invention relates to coating dispensing apparatus (hereinafter sometimes “spray guns” or “guns”) for electrostatically aided atomization and dispensing of coating materials, and particularly to high magnitude potential generators for such guns.
Various different types of spray guns having in-gun high magnitude potential generators are known. There are, for example, the manual spray guns illustrated and described in the following listed U.S. Pat. Nos. and published applications: 2003/0006322; 6,460,787; 6,276,616; 5,178,330; D325,241; D318,712; 5,022,590; 4,993,645; 4,934,607; 4,934,603; 4,911,367; 4,747,546; 4,574,092; 4,529,131; 4,508,276; 4,498,631; 4,433,003; 4,331,298; 4,290,091; 4,258,409; 4,248,386; 4,219,865; 4,165,022; 4,020,393; 3,991,710; 3,791,579; 3,731,145; 3,687,368; 3,673,463; 3,651,354; and, 3,608,823; and British Patent 1,387,632. Reference is here also made to U.S. Pat. Nos. 6,562,137; 6,423,142; 6,144,570; 5,978,244; 5,159,544; 4,745,520; 4,485,427; 4,481,557; 4,324,812; 4,187,527; 4,075,677; 3,894,272; 3,875,892; 3,851,618; and, 3,567,996. Reference is also made to U.S. Ser. No. 11/153,989 filed Jun. 16, 2005, titled In-Gun Power Supply Control, and assigned to the same assignee as this application. The disclosures of these references are hereby incorporated herein by reference. This listing is not intended to be a representation that a complete search of all relevant art has been made, or that no more pertinent art than that listed exists, or that the listed art is material to patentability. Nor should any such representation be inferred.
According to an aspect of the invention, a combination includes a power supply and an enclosure for housing the power supply. The enclosure has a wall and a valve providing access through the wall to evacuate the enclosure. The components of the power supply are subject to the atmosphere within the enclosure.
According to another aspect of the invention, the power supply and enclosure are incorporated into an electrostatic spray gun.
Illustratively according to the invention, the power supply is selected from the group of power supplies including electrogasdynamic supplies, supplies including gas turbine driven generators or alternators, supplies including piezoelectric generators, supplies including triboelectric generators, such as Van de Graaff generators, supplies including transformers for transforming AC line voltage variations and multipliers, and supplies including a low voltage DC supply, an inverter, a transformer and a multiplier.
Illustratively according to the invention, the power supply comprises a supply including a transformer and a multiplier.
Illustratively according to the invention, the components of the power supply which are housed in the enclosure include the multiplier.
Further illustratively according to the invention, the apparatus includes a high dielectric constant gas or mixture of gases. The gas or mixture of gases is introduced into the enclosure after evacuation of the enclosure.
Illustratively according to the invention, the high dielectric constant gas or mixture of gases comprises sulfur hexafluoride.
Illustratively according to the invention, the electrostatic spray gun comprises a somewhat pistol grip-shaped handle and a barrel extending from the handle, the enclosure forming at least a part of the barrel.
The invention may best be understood by referring to the following detailed description and accompanying drawings which illustrate the invention. In the drawings:
The invention can be adapted for use with any of a number of different power supplies and power supply configurations. By way of example, but certainly not by way of limitation, these include: AC line supply/transformer/multiplier and internal (for example, battery) or external low voltage DC supply/inverter/transformer/multiplier supplies of the general types illustrated and described in the above referenced U.S. Pat. Nos. 4,331,298, 4,165,022, 3,731,145, 3,687,368, and 3,608,823, and U.S. Ser. No. 11/153,989 (see FIGS. 1-3); electrogasdynamic supplies of the general type illustrated and described in the above referenced U.S. Pat. Nos. 4,574,092, 4,498,631, 4,433,003, 4,020,393, 3,991,710, 3,791,579, 3,673,463, and 3,651,354 (see
Without any intention to be limited in the types of power supplies to which the present invention can be adapted, the invention will be described in connection with an external low voltage DC supply/inverter/transformer/multiplier supply of the type illustrated and described in the above referenced U.S. Ser. No. 11/153,989. Referring now particularly to
An externally generated low DC voltage of, for example, ≦24 VDC, provided on a conductor 117 is converted by oscillator circuit 104, driver circuit 106 and switch(es) 108 to an AC signal across a primary winding of transformer 110. The transformer 110 produces across its secondary windings an AC voltage of, for example, 5 KV that is then rectified and multiplied in voltage multiplier 112 to provide at an output terminal 123 of voltage multiplier 112 a voltage suitable for efficient electrostatic application of coating material, for example, negative 60-90 KV DC. The high voltage generator circuit must be made as small and lightweight as possible to facilitate manipulation of the hand-held electrostatic spray gun 102 in which it is mounted. The components of power supply 100 must therefore be placed extremely close together. This raises the possibility of electrical breakdown. Heretofore, it was common practice to pot certain elements of this assembly, for example, PC board 119 and components 110 and 112, using high dielectric strength potting compound in order to provide dielectric insulation for certain components of the power supply 100. The potting compounds, when cured, exhibit dielectric strengths in the 400-500 volts/mil (about 15.7 KV/mm about 19.7 KV/mm) range, which is suitable to protect against dielectric breakdown between components at different electrical potentials, assuming that special components and manufacturing techniques are observed and that care is taken in the design of the assembly.
According to a first illustrated embodiment, PC board 119 and components 104, 106, 108, 110 and 112 that previously would have been potted with potting compound are mounted in a vessel 130 of generally right circular cylindrical configuration. The vessel 130 is closed by flat, part-spherical, or other suitable configuration ends. The configurations of the vessel 130 sidewall(s) and ends need be such as to provide the necessary strength to withstand evacuation and optionally pressurization. Referring now particularly to
High voltage arcing is initiated by ionization of an insulating medium. A vacuum represents the absence of any ionizable insulating medium. Therefore, according to another embodiment, the vessel 130 containing one or more of the components 104, 106, 108, 110, 112, 119 that previously would have been potted with potting compound is evacuated from source 142 through its valve 132 down to a few millibars and the valve 132 is then closed to maintain the atmosphere inside vessel 130 at relatively high vacuum. A thus-evacuated vessel 130 can provide up to 2,000 volts per 0.001 inch (2,000 volts/mil) (about 79 KV/mm) dielectric strength.
Vessel 130 and its end closure cap 140 are constructed from any suitable material. Materials that are highly gas-impermeable and will not outgas significantly are preferred because they will not contribute leaked and/or outgassed components, such as volatile organic solvents, plasticizers and the like, back into the atmosphere inside vessel 130 once it is pumped down to relatively high vacuum, whether or not it is then repressurized with high dielectric strength gas. Such materials include certain ceramics, certain glasses, and certain very rigid resins and polymers. In the illustrated embodiment, a circuit board 146 on which components of high voltage multiplier 112 are mounted also functions as a spacer or standoff for component 112 and any of components 104, 106, 108, 110 that are mounted to it. Alternatively, one or more spacers constructed from, for example, the same or similar materials as vessel 130 can be placed around the components 104, 106, 108, 110, 112, 119 that are placed into the vessel 130 to maintain relatively uniform spacing between the components 104, 106, 108, 110, 112, 119 and the vessel 130 sidewall(s) 134.
Potted high magnitude power supply components typically require special fabrication processes to maximize adhesion of the potting compound to component surfaces. Component spacing and special soldering techniques must also be observed in order to reduce the occurrence of high dielectric stress points which would promote electrical breakdowns and discharges to adjacent components. Many potting compounds currently in use are susceptible to thermal stress which also calls for precise fabrication techniques. Use of the evacuated vessel 130 or vessel 130 evacuated and then repressurized with higher dielectric strength gas reduces or eliminates potting processes and associated manufacturing complexities and enhances reliability.
Use of the evacuated vessel 130 or vessel 130 evacuated and then repressurized with higher dielectric strength gas also reduces the weight of the power supply 100 and therefore the overall weight of the gun 102. This reduces operator fatigue, makes the gun 102 more maneuverable, and so on.
