US3384050A

US3384050A - Electrostatic coating system

Info

Publication number: US3384050A
Application number: US657631A
Authority: US
Inventors: Marcel A R Point
Original assignee: Sames Sa De Machines Electrost
Current assignee: Machines Electrostatiques SA
Priority date: 1963-02-19
Filing date: 1967-08-01
Publication date: 1968-05-21
Anticipated expiration: 1985-05-21
Also published as: DE1577920A1; US3342415A; FR88538E; DE1577920B2; FR1361917A; GB1053514A

Description

y 21, 1963 M. A. R. POINT 3,384,050

ELECTROSTAT I C COATING SYSTEM Filed Aug. 1, 1967 United States Patent 3,384,050 ELECTROSTATIC COATING SYSTEM Marcel A. R. Point, Grenoble, France, assignor to SAMES, Societe Anonyme de Machines Electrostatiques, Paris, France, a French joint-stock company Continuation-impart of application Ser. No. 345,723, Feb. 18, 1964. This application Aug. 1, 1967, Ser. No. 657,631 Claims priority, application France, Feb. 19, 1963,

10 Claims. :1. 118-626) ABSTRACT OF THE DISCLOSURE circular edge and is directed by the electrostatic field onto the surface being coated.

CROSS REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of copending application Ser. No. 345,723, filed Feb. 18, 1964, by Marcel A. R. Point, now Patent No. 3,342,415.

BACKGROUND OF THE INVENTION This invention relates to coating systems and more particularly to such systems in which the coating substances are applied to the articles to be coated through the agency of an electrostatic field.

Electrostatic coating systems of this general class have gained wide acceptance in recent years owing to the outstanding advantages they present over more conventional systems which involve spraying in the absence of an electric field. These advantages include more positive guidance of the particles of coating substance toward and onto the surfaces of the work being coated, whereby loss of coating substance is reduced to a minimum; creation of a strong electrostatic bond between the work surfaces and the coating substance, whereby subsequent heating or other bonding treatments can in many cases be dispensed with; more uniform coating thickness; and other advantages.

A wide variety of coating substances may be effectively applied through the use of electrostatic techniques. Because of difficulties encountered heretofore in the application of pulverulent substances, such as enamels, plastic compositions and other solid coating materials, the most widely accepted electrostatic coating processes have involved the use of paints or other liquid substances. One of the reasons for the deficiencies in the application of powdered materials has been that it is considerably more difficult to impart a high and uniform electric charge to particles of powder than to a liquid. The physical phenomena through which powder particles and liquid droplets acquire charges are quite different. A liquid becomes charged by conduction, that is, through electric contact with a high voltage electrode, whereas a powder obtains its charge primarily through ionization. Such ionic charging is inherently less eflicient than conduction charging, with the result that the build-up of charges on the indi- 3,384,050 Patented May 21, 1968 vidual particles of the powder has been insuflicient in many instances.

Some of the foregoing deficiencies may be overcome through the use of an electrostatic coating system of the type disclosed in Point et al. U.S. Patent 3,263,127, granted July 26, 1966. Systems of this type include a spray nozzle or gun to which the pulverulent coating substance is delivered from a store thereof by means of a high velocity air stream. An electrode associated with or forming part of the nozzle is positioned so that the particles of coating substance suspended in the air stream must pass in a thin sheet-like cloud over or close to the electrode as they issue from the nozzle outlet. The electrode is connected to a high D.C. potential, illustratively of the order of one hundred or more kilovolts, which is supplied by an electrostatic generator or other suitable source. The particles are thus charged to a corresponding potential and are positively guided over the lines of force of the electrostatic field created between the electrode and the surface of the work, which is usually at ground potential. The particles are bonded to the work surface by electrostatic attraction.

Another known arrangement, as disclosed, for example, in Barford et al. U.S. Patent 3,248,253, granted Apr. 26, 1966, employs a relatively stationary, fluidized bed of coating powder. Air under pressure is injected upwardly through the perforate base of a tank containing the powder, and point electrodes connected to a high DC potential project into the tank to charge the fluidized powder particles. The articles to be coated, preferably at ground potential, are brought into position in or just above the tank such that their surfaces attract the fluidized particles. The particles are bonded to the surfaces by electrostatic attraction.

The various systems just outlined perform satisfactorily. However, fluidized beds and similar equipment are not sufficiently portable for many coating applications, while the structure utilized to form the thin sheetlike cloud of particles in electrostatic spray guns represents a potential source of clogging and includes other deficiencies.

In another known system, the coating powder is contained in an overhead tank having a perforate bottom and a point electrode projecting downwardly therefrom. The tank is vibrated to discharge the particles through the perforate bottom and past the electrode, thus charging the particles and causing the charged particles to travel under the combined actions of gravity and the electrostatic field onto the surfaces of the work below the tank. For a more detailed discussion of representative systems of this type, reference may be had, for example, to Paton U.S. Patent 3,058,443, granted Oct. 16, 1962.

This last arrangement is advantageous in that it dispenses with the use of pressurized air. However, the need for vibrating the tank introduces another complication. Moreover, the satisfactory operation of the system depends to some extent on a somewhat critical granulometry of the coating powder as well as on a precise adjustment of the height of the tank above the work, to ensure that the particles follow the prescribed path under the resultant of gravity and electrostatic forces.

One general object of this invention, therefore, is to provide a new and improved system for applying a coating substance to work through the agency of an electrostatic field.

More specifically, it is an object of the present invention to provide an electrostatic coating system which enables the realization of a smooth and uniform discharge of the coating substance without the possibility of clogging the discharge member of the feed passage leading thereto.

Another object of the invention is to provide a coating system of the character indicated in which solid powder coating substances are electrostatically applied to the work in a rapid and straightforward manner.

A further object of the invention is to provide such coating system which ensures the smooth discharge of a wide variety of coating substances having widely varying granulometric ranges.

Still another object of the invention is to provide an electrostatic coating system in which the coating is applied to objects of comparatively intricate shapes without the need for delicate vertical adjustments of the coating a-pparatus.

Another object of the invention is to provide an electrostatic coating system which is economical to manufacture and thoroughly reliable in operation.

In several advantageous embodiments of the invention, there is provided an electrostatic coating system which utilizes the simultaneous and combined actions of centrifugal and electrostatic forces to effect in a simple and economical manner a smooth and positive deposition of a finely divided pulverulent coating substance on the surfaces of the work. In these embodiments, there is provided a store of the coating substance, a disk-like discharge member driven in rapid rotation adjacent the surface of an article to be coated, a feed passage leading from the store to a point spaced from the disk-like member, so as to feed the substance thereto for discharge by centrifugal force from the periphery of the member toward the surface of the article, and electrode means associated with or forming part of the periphery of the disk for charging the particles as they are discharged therefrom and creating an electrostatic field for guiding the charged particles toward the article.

Exemplary embodiments of the invention will now be described, for purposes of illustration but not of limitation, with reference to the accompanying drawings, wherein:

FIGURE 1 is a simplized, partially schematic overhead view of portions of an electrostatic coating system according to one illustrative embodiment of the invention, as used in the enamelling of the internal surface of a bathtub; and

FIGURE 2 is a diagrammatic view, with certain parts shown in section and others in elevation, of an electrostatic coating system in accordance with the embodiment of FIGURE 1.

Referring to FIGURES 1 and 2, there is illustrated a bathtub 1 which is to have its inner concave surface coated with an enamelling composition. The bathtub 1 is made of an electrically conductive material, for instance cast iron, and is electrically connected to ground. An overhead monorail conveyor 2 extending above the bathtub I has a trolley 4 horizontally movable thereover by means of rollers 3. The trolley 4 supports a frame shown schematically at 12 which is arranged for vertical traversing movement relative to the trolley along guide rails 5. These guide rails extend in a downward direction from the trolley 4 and are provided with rollers 6 which engage the adjacent faces of the frame 12.

A tubular arm 7 projects downwardly from the frame 12. The arm 7 accommodates a shaft 8 which is rotatably supported therein by suitable bearings (not visible in FIGURES 1 and 2). In the illustrated embodiment, the arm 7 and the shaft 8 extend in a vertical direction, although in other arrangements these components may be provided with various other orientations in accordance with the disposition of the surface to be coated. Secured to the projecting lower end of shaft 8 is a generally diskshaped discharge member 10. The disk member 10 is preferably made of conductive material and is provided with a comparatively sharp peripheral edge. The member 10 includes a circumferential series of vertically extending holes 11 formed through it near its periphery. The upper end of the shaft 8 is coupled to an electric motor (not shown) carried by the frame 12.

The disk member 10 is electrically connected, as by a slip ring connection (not shown), to the negative terminal of a source 15 of high DC. voltage, the other terminal of which is grounded. The source 15 illustratively comprises an electrostatic generator having an output voltage of the order of one hundred or more kilovolts.

Supported by the frame 12 is a hopper 9 which contains a store 14 of coating substance. The coating substance illustratively comprises a suitable enamelling powder. The bottom outlet of the hopper 9 is connected to a vertical tube 13 which extends downwardly therefrom and terminates a short distance above the upper surface of disk member 10. Suitable valve means (not shown) are provided for controlling the discharge of the coating substance from the hopper 9 through the pipe 13.

In operation, power is applied to rotate the shaft 8 and the disk member 10 at high speed, and a high D.C. voltage is applied to the member 10 from the generator 15. The coating substance 14 flows by gravity from the hopper 9 down the pipe 13. Because of the substantial vertical length of the pipe and the consequent pressure head, the substance reaches the upper surface of the member 10 under substantial pressure. As the substance drops onto the member 10, it is projected outwardly by centrifugalforce and travels in a thin sheet over the upper surface of the member. A portion of the substance passes through the holes 11, while the remainder moves past the sharp peripheral edge of the member. During such travel, any small masses or caked fragments that may be present in the coating substance are broken up by centrifugal force and are discharged in substantially fully divided form.

The powder particles, upon moving past the sharp electrode formed by the peripheral edge of the discharge member 10 become charged to a high DC. potential. The thus charged material follows the lines of force, somewhat as indicated by the arrows in FIGURE 2, of the electrostatic field created between the member 10 and the inner surfaces of the bathtub 1. The portion of the coating substance which passes through the holes 11 for the most part is directed toward the lower, substantially horizontal surface of the bathtub, While the remaining portion moves toward the inner side walls of the tub. If desired, the frame 12 is continuously or intermittently reciprocated horizontally over the monorail 2 or vertically along the rails 5 along the length or depth of the tub. With this arrangement, the uniformity of the coating on the tub surfaces is further enhanced.

In a modified embodiment, there may be provided a plurality of axially spaced disk members of the type disclosed in copending application Ser. No. 345,723 referred to above. Such a multiple-disk arrangement is advantageous, among other respects, in the simultaneous discharge of different coating substances or ingredients on a common work surface. As an example, in one arrangement two of the disk members serve to discharge a thermosetting resin powder, while the third disk member acts simultaneously to discharge a setting agent or polymerization catalyst therefor.

In the illustrated embodiment of the invention, the force of gravity and centrifugal force are used jointly with the force of the electrostatic field in order to accomplish a smooth and positive discharge of finely divided pulverulent coating particles upon the surface of the work without an entraining fiuid stream. In other good arrangements, the flow of the particles is further enhanced through the use of a carrier gas such as compressed air, as more fully described in US. Patent 3,263,127 referred to heretofore. The particles form a thin sheet on the conical surface of the discharge member and are discharged from the circular edge of the member by centrifugal force. This sheet has a large dimension which is coextensive with the circular edge and a small dimension extending in a plane normal to the edge. Such a system is of particular value in connection with the coating of the internal surfaces of recessed articles, such as a bathtub, for example. The centrifugal force field created by the revolving discharge member inherently presents a symmetry of revolution about the axis of rotation, so that maximum eificiency is obtained in cases where the surface of the work substantially surrounds the discharge member. -In some cases, the system is employed to simultaneously coat a plurality of articles, each of which includes a surface which is flat or of other shape, by arranging the articles in a circumferential annular array around the periphery of the discharge member. In other embodiments, the system is advantageous for applications where the work surfaces are not disposed around the discharge member, as by provid ing means for masking a portion of the circumference of the member with a shield or bafile, for example, or for other applications.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described, or portions thereof, it being recognized that various modifications are possible within the scope of the invention claimed.

What is claimed is:

1. Electrostatic coating apparatus comprising, in combination, a store of solid pulverulent coating material, a rotary discharge member having a sharp peripheral edge of circular configuration supported adjacent a surface of the work to be coated, means defining a feed passage leading from said store to a point spaced from said discharge member for feeding the pulverulent coating material thereto, said discharge member including an open, unobstructed surface facing the feed passage for receiving the pulverulent material therefrom, means for rotating said discharge member at a speed suflicient to permit the free discharge and dispersion of the pulverulent material by centrifugal force from said circular edge, a source of high DC. potential, and means connected to said source and operatively associated with said circular edge for electrically charging the particles of material as they are discharged from said member, to thereby produce an electrostatic attraction between said particles and the surface being coated.

2. Electrostatic coating apparatus comprising, in combination, a store of solid pulverulent coating material, a rotary discharge member having a sharp peripheral edge supported adjacent the work to be coated, means defining a feed passage open to the atmosphere and leading from said store to a point spaced from said discharge member for feeding the pulverulent coating material thereto, said discharge member including an open, unobstructed surface facing the feed passage for receiving the mass of material therefrom, means for rotating said discharge member at a speed sufiicient to shape the mass of material being fed to said surface into a thin pulverulent sheet of substantially uniform thickness and to permit the free discharge and dispersion of said'sheet by centrifugal force from said peripheral edge, a source of high DC. potential, and means connected to said source and operatively associated with said peripheral edge for electrically charging the particles of material from said sheet as they are discharged from said member, to thereby produce an electrostatic attraction between said particles and the surface being coated.

3. Electrostatic coating apparatus comprising, in combination, a store of solid pulverulent coating material, a rotary discharge member having a sharp peripheral edge of circular configuration supported adjacent a surface of the work to be coated, means defining a feed passage leading from said store to a point spaced from said discharge member for feeding the pulverulent coating material thereto, said discharge member including an open, unobstructed surface facing the feed passage for receiving the mass of material therefrom, means for rotating said discharge member at a speed sufficient to shape the mass of material being fed to said surface into a thin pulverulent sheet of substantially uniform thickness and to permit the free discharge and dispersion of said sheet by centrifugal force from said circular edge, said sheet having a large dimension coextensive with said circular edge and a small dimension in a plane normal to said edge, a source of high DC. potential, and means connected to said source and operatively associated with said circular edge for electrically charging the particles of material from said sheet as they are discharged from said member, to thereby produce an electrostatic attraction between said particles and the surface being coated.

4. Electrostatic coating apparatus of the character set forth in claim 3, in which said open, unobstructed surface is of generally conical configuration.

5. Electrostatic coating apparatus comprising, in combination, a store of solid pulverulent coating material, a rotary discharge member having a sharp peripheral edge of circular configuration supported adjacent va surface of the work to be coated, means defining a feed passage open to the atmosphere and leading from said store to a point spaced from said discharge member, to feed a con densed mass of the pulverulent coating material thereto by gravity without an entraining fluid stream, said discharge member including an open, unobstructed surface facing the feed passage for receiving the mass of material therefrom, means for rotating said discharge member at a speed sufficient to shape the mass of material being fed to said surface into a thin pulverulent sheet of substantially uniform thickness and to permit the free discharge and dispersion of said sheet by centrifugal force from said circular edge, a source of high DC. potential, and means connected to said source and operatively associated with said circular edge for electrically charging the particles of material from said sheet as they are discharged from said member, to thereby produce an electrostatic attraction between said particles and the surface being coated.

6. An electrostatic coating system comprising, in combination, a store of solid pulverulent coating substance, at least one rotary discharge member of generally diskshaped configuration adjacent the surface of the work to be coated and having a sharp peripheral edge of circular contour, means defining a feed passage leading from said store to a point spaced from said discharge member, to feed the coating substance thereto by gravity, said memher having an open, unobstructed upper face for receiving said coating substance from the downwardly leading feed passage and for permitting the free discharge and dispersion of said substance in divided form by centrifugal force from the periphery of said member, a source of high DC. potential, and means connected to said source and operatively associated with said circular edge of the discharge member for electrically charging the particles of said coating substance as they are discharged from said member, to thereby produce an electrostatic attraction between said particles and the surface being coated.

7. An electrostatic coating system comprising, in combination, an overhead store of solid pulverulent coating substance, a rotary discharge member of circular, generally disk-line configuration having a sharp peripheral edge mounted below said store adjacent the surface of the work to be coated, means defining a feed passage leading downwardly from said store to a point overlying said discharge member, to feed the coating substance thereto by gravity, said member having an open, unobstructed upper face for receiving said coating substance from the downwardly leading feed passage and for permitting the free discharge and dispersion of said substance in divided form by centrifugal force from the periphery of said member, a source of high D.C. potential relative to the work to be coated, and means for supplying said potential to said discharge member to electrically charge the particles of said coating substance as they are discharged therefrom,

to thereby produce an electrostatic attraction between said particles and the surface being coated.

8-. An electrostatic coating system comprising, in combination, an overhead store of solid pulverulent coating substance, at least one disk-shaped discharge member having a circular peripheral edge mounted below said store adjacent the surface of the work to be coated, said discharge member being supported for rotation about a vertical axis and including a series of perforations extending in directions parallel to said axis, means defining a feed passage leading downwardly from said store to a point overlying said discharge member, to feed the coating substance thereto by gravity, said member having an open, unobstructed upper face for receiving said coating substance from the downwardly leading feed passage and for permitting the free discharge and dispersion of said coating substance by centrifugal force from the periphery of said member, means for rotating said member to dis charge said substance in divided form by centrifugal force therefrom, a portion of the discharged substance passing through said perforations, a voltage source, and means connected to said source for electrically charging the particles of said coating substance as they are discharged from said member, to thereby produce an electrostatic attraction between said particles and the surface being coated.

9. An electrostatic coating system for applying a coating substance to the internal surface of a recessed article, said system comprising, in combination, an overhead frame, a supply of solid pulverulent coating substance carried by said frame, an elongated supporting structure depending from said frame, means including a generally disk-shaped discharge member having a sharp peripheral edge of circular configuration mounted on the lower end of said supporting structure for rotation about a vertical axis, said discharge member being positioned adjacent the internal surface of the article to be coated, said supporting'structure defining a feed passage leading downwardly from said store to a point overlying said discharge member, to feed said coating substance thereto by gravity under a substantial pressure head, said member having an open, unobstructed upper face for receiving said coating substance from the downwardly leading feed passage and for permitting the free discharge and dispersion of said substance in divided form by centrifugal force, a

source of high DC. potential relative to said article, and means connected between said source and said discharge member for applying said high DC. potential thereto, to electrically charge the particles of said substance as they are discharged from said discharge member, to thereby produce an electrostatic attraction between said particles and the surface being coated.

10. An electrostatic coating system comprising, in combination, an overhead frame, a store of solid pulverulent coating substance carried by said frame, a supporting structure depending from said frame, means including a generally disk-shaped discharge member having a sharp peripheral edge of circular configuration mounted on the lower end of said supporting structure for rotation about a vertical axis adjacent the surface of the work to be coated, said supporting structure defining a feed passage leading downwardly from said store to a point overlying said discharge member, to feed said coating substance thereto by gravity under a substantial pressure head, said member having an open, unobstructed upper face for receiving said coating substance from the downwardly leading feed passage and for permitting the free discharge and dispersion of said substance in divided form by centrifugal force from the periphery of said member, means operatively associated with said frame for im parting both vertical and horizontal reciprocatory movements thereto, to impart similar movements to said store, said supporting structure and said discharge member during the discharge of said substance therefrom, a source of high DC. potential, and means connected to said source for electrically charging the particles of said substance as they are discharged from the periphery of said discharge member, to thereby produce an electrostatic attraction between said particles and the surface being coated.

References Cited UNITED STATES PATENTS 1,958,406 5/1934- Darrah 118-629 XR 3,039,890 6/1962 Reindl 118636 XR 3,058,443 10/1962 Paton 118-622 3,248,253 4/1966 Barford et al. 118622 XR CHARLES A. WILLMUTH, Primary Examiner. PETER FELDMAN, Examiner.

Disclaimer 3,384,050.]l!arcel A. 1?. Point, Grenoble, France. ELECTROSTATIC COATING SYSTEM. Patent dated May 21, 1968. Disclaimer filed April 29, 1977, by the assignee, Societe Anonyme De lilac/Lines Electrostatigues. Hereby enters this disclaimer to

claims

1, 2, 3 and 4 of said patent.

[Official Gazette October 18, 1.977.]