US 3476319 A
Description (OCR text may contain errors)
c. R. THATCHER 3,476,319 ELECTROSTATIC POWDER-COATING APPARATUS Nov. 4, 1969 o igi'nal Filed May 10, 1965 vm hm \2 T (O in INVENTQR. CHARLES R. 'THATCHER' United States Patent 'Int. Cl. 1305b /08 7 US. Cl. 239 15 6 Claims ABSTRACT .OF THE DISCLOSURE v Electrostatic powder-coating apparatus wherein a suspension of powder in a gaseous medium is discharged as a spray from a nozzle of insulating material the front face of which lies rearwardly'of the plane of the nozzle orifice. An electrode surrounds the'nozzle body, and a spray-spreading deflector is mounted forwardly of the nozzle orifice innon-obstruction relation thereto.
This is a continuation of Ser. No. 549,044 filed May 10, 1966, now abandoned.
This invention relates to electrostatic powder-coating apparatus and isparticularly concerned with an improved type of nozzle for forming a powder spray and electrically charging the powder particles. Inpowder coating-operations it has been a common practice to suspend the powder in a gaseous mediumand discharge the suspension through a nozzle'so constructed as to form the discharged stream of suspension into a conical spray directed toward the article or articles to be coated. One form of nozzle heretofore used has employed a deflector positioned to be impinged upon by the emerging suspension stream and serving to spread the stream and deflect it into contact with a forwardly flaring, spread-limiting nozzle-portion the rim of which functions, by virtue of a connection to a high voltage source, as an electrode to charge the powder particles and cause them to be electrostatically attracted to the work. Where safety against sparking is desired, the nozzle has been formed of insulating material, and the outer surface of the nozzle, including the outer surface of its forwardly flaring portion, has been coated with an electrically resistive coating the forward edge of which serves as the electrode. .While such nozzles have proven generally satisfactory and have had wide use, difficulty sometimes arises as a result of powder adhering to the inner face of the forwardly flaring nozzle portion. Even if such adherence is only' temporary, it is still objectionable; since, accumulated powder dislodged from such surface frequently leaves in the form of relatively large flakes which may cause disuniformities in the thickness of the deposited coating.
The present invention springs from the discovery that it is not necessary to employ a structural element, such as above mentioned forwardly flaring nozzle portion, to limit the spreading of the spray caused by the deflector. In a nozzle embodying the invention, the stream of powder suspension emerging from an outlet in a nozzle body of insulating material impinges upon a spreading deflector as in prior nozzles above described; but instead of forming the front of the nozzle body to limit the spreading action, it is formed so that substantially its entire front face lies rearwardly of said outlet and out of the paths of spray particles deflected by the deflector. Preferably the front face of the nozzle body is a rearwardly flaring, frustoconical surface terminating in a circular edge coincident with the forward edge of a resistive coating on the exterior of the nozzle body. The deflector is supported from the nozzle body by means which may be adjustable to vary the axial position of the deflector,
3,476,319 Patented Nov. 4, 1969 thereby permitting control of the degree of spray-spreading effected by the deflector.
Other features of the invention will become apparent from the following more detailed description and from the drawing, in which:
The figure is an illustration, partially diagrammatic in character, of an electrostatic powder-coating apparatus including a spray gun parts of which are shown as broken away to illustrate the interior construction.
The apparatus shown includes a gun 10 comprising an elongated tubular barrel 11 enclosingtwo longitudinally extending tubes 12 and 13 supported in plugs 14 and 15 at the front and rear ends of the barrel. Desirably, the barrel, the tubes, and the plugs are all formed of insulat ing material. At its front end, the tube 12 projects outwardly beyond the plug 14 where it supports a nozzle designated in its entirety by the reference numeral 16. At its rear end,'the tube 12 is connected through a fitting 17 to a supply conduit 18, conveniently a flexible plastic tube, leading from a means 19 for delivering a suspension of the powder in a gaseous medium. Details of the means 19 are not illustrated, as various forms of such means are well known.
The tube 13 houses near its front end a resistor 21 the rear end of which is electrically connected to a flexible cable 22 secured to the rear plug 15 by an appropriate fitting 23 and extending therefrom to the ungrounded terminal of a high-voltage source or power pack 24 the other terminal of which is grounded. Received in the front end of the tube 13 is an electrically conductive plug 26 which is electrically connected to the front end of the resistor 21 and which carries at its front end a flexible brush 27 making electrical contact with the nozzle 16.
The nozzle 16, with which this invention is primarily concerned, comprises a body 28 of insulating material provided with a through passage having a rear portion 29 which snugly receives the front end of the tube 12 and a front portion 30 of smaller diameter through which the powder suspension is discharged. Substantially the entire front face of the nozzle body is located rearwardly with respect to the plane of the outlet of the passage-portion 30. Most desirably, the front end of the nozzle body is enlarged to provide a head 31 of circular shape as viewed from the front of the gun, the front face of such head being a frustoconical surface 32 which flares rearwardly from the periphery of the outlet to the periphery of the head. Spaced from the nozzle outlet and coaxial therewith is a suspension-spreading deflector 33 desirably in the form of a circular disc of insulating material preferablysupported from the nozzle body by an adjustable means which will permit variation in the axial position of the deflector. As shown, such means comprises a wire helix 34 received at its rear end in the passage-portion and terminating at its front end in a substantially plane, approximately complete wire-turn received in a peripheral rabbet groove in the rear edge of the deflector 33. The deflector may be secured to the end turn of the helix by any apropriate type of cement. Desirably, the rear portion of the helix 34 has a constant pitch corresponding to that of a shallow helical groove 36 formed in the wall of the passage-portion 30. Between the constant-pitch portion of the helix 34 and the plane end turn which receives the deflector 33, the helix is open-wound to permit the radial escape of the powder suspension discharged from the passage-portion 30'.
Rearwardly from the peripheral edge of the front surface 32 of the head 31, the exterior of the nozzle body is coated with a conductive coating of high electrical resistance. Such coating may be of the type disclosed in US. Patent 3,021,077 and, as shown (with thickness exaggerated) at 37, extends rearwardly over the outer sur- 3 I face of the nozzle body far enough to be engaged by the brush 27.
In operation of the apparatus described, the powder suspension supplied by the means 19 is conducted through the conduit 18 and tube 12 into' the nozzle passage 29-30, from which it is discharged against the deflector 33, such deflector serving to spread the stream of powder suspension into a divergent conical spray. The coating 36, which is charged to high electrical potential through the resistor 21 and cable 22, terminates forwardly at a circular edge coincident with the outer periphery of the frustoconical surface 32 on the nozzle-head 31. The front edge of the coating serves as a discharge electrode by means of which the sprayed particles are electrically charged and become clectrostatically deposited on spray-receiving article maintained at ground potential. Since all portions of the nozzle-body are located rearwardly of the plane of the nozzle-outlet, and since the electrostatic forces urge the sprayed particles forwardly away from the'nozzle, the
nozzle does not accumulate any coating of the powder.
The rear portion of the helix 34 fits the passage-portion 30 with suflicient tightness to prevent accidental displacement of the deflector but not so tightly as to interfere with axial adjustment of the helix and the deflector secured to it. If, as is preferred, the rear end of the helix is received in a helical groove, axial adjustment of the deflector can be effected by rotating it and the helix,
What is claimed is: 1. In an electrostatic powder-coating apparatus wherein a suspension of powder in a gaseous medium is formed into a spray for electrostatic deposition on an article to be coated, a spray-forming nozzle means comprising a body of electrically insulating material provided with a passage extending through it, said passage having at its rear end an inlet connected to a source of the suspension and at its front end a forwardly directed orifice through which the suspension is discharged as a spray moving axially with respect to the orifice, a deflector for spreading said spray, means for supporting said deflector in alignment with said passage extending forwardly from said nozzle body beyond said orifice and not obstructing the flow of suspension in said passage, said means extending far enough forwardly from the orifice not to prevent the axial flow of suspension as it leaves the orifice, substantially the entire front face of said nozzle-body being located rearwardly of the plane of the orifice, an electrode surrounding said nozzle-body, said electrode terminating at its front end in an edge located rearwardly of said plane, and means for creating a high difference of electrical potential between said electrode and an article being coated whereby to cause charging of the powder particles in the spray and the electrostatic deposition of such charged particles on the article.
2. Apparatus as set forth in claim 1 wherein said means for supporting said deflector engages the walls of the passage and is adjustable to vary the distance between said deflector and said orifice.
3. Apparatus as set forth in claim 2 wherein said means for supporting said deflector is a wire helix received at its rear end within said passage and secured at its front end to said deflector, that portion of said helix lying adjacent the deflector being open-wound to permit the lateral escape of its suspension, and the front end of said passage having in its wall a helical groove screw-threadedly. receiving the rear end of the helix. 1
4. Apparatus as set forth in claim 1 wherein said deflector is supported from the nozzle body through a wire helix having a rear end snugly received in said passage and an open-wound front endportion secured to the deflector.
5. In an electrostatic powder-coating apparatus wherein a suspension of powder in a gaseousmedium is formed into a spray for electrostatic deposition on an article to be coated, a spray-forming nozzle means comprising a body of electrically insulating material having a frustoconical front face and provided with a passage extending through it, said passage having at its rear end an inlet connected to a source of the suspension and at .its front end a forwardly directed orifice through which the suspension is discharged as a stream, a deflector disposed in alignment with said passage and spaced from said orifice in the direction of discharge therefrom in a position to spread the suspension-stream emerging from the orifice, substantially the entire front face of said nozzle-body being located rearwardly of the plane of the orifice, an electrode surrounding said nozzle-body, said electrode terminating at its front end in an edge located rearwardly of said plane and substantially coincident with the outer edge of said front face, and means for creating a high difference of electrical potential between said electrode and an article being coated whereby to cause charging of the powder particles in the spray and the electrostatic deposition of such charged particles on the article.
6. Apparatus as set forth in claim 3 wherein said electrode is a coating of electrically resistive material applied to the peripheral surface of the nozzle-body.
References Cited UNITED STATES PATENTS 3,021,077 2/1962 Gauthier 239- 1s 3,111,266 11/1963 Axelson et al. 239-15 3,248,606 4/1966 Fraser 239 15 x 3,263,127, 7/1966 Point et al 239-3 x EVERETT 'W. KIRBY, Primary Examiner US. 01. X.R.