US2884341A - Electrostatic spray coating method - Google Patents

Description

April 28, 1959 J. w. JUVINALL 2,834,341

ELECTROSTATIC SPRAY comma METHOD Filed March '7, 1955 2 Sheets-Sheet 1 F/ 2 g INVENTOR.

JAMES w. JUVl/VALL April 28, 1959 J. W. JUVINALL ELECTROSTATIC SPRAY comma METHOD ZSheets-Shet 2 Filed March 7, 1955 lIIlll II- II E r INVENTOR. JAMES W. JUl/l/VALL BY A fforneys United States Patent Q ELECTROSTATIC SPRAY COATING METHOD James W. Iuvinall, Indianapolis, Ind., assignor to Ransburg Electro-Coating Crp., Indianapolis, Ind., a corporation of Indiana Application March 7, 1955, Serial No. 492,379

2 Claims. (Cl. 117--93) This invention relates to a method of and apparatus for spray coating articles and more particularly to a method of and apparatus for electrostatically spray coating articles where the distribution of the coating material on the articles being coated may be controlled in a desired manner by coordination of the movement of the spray source with the movement of the articles.

Difiiculty has been experienced in obtaining a uniform coating on articles of relatively large volume with automatic spray apparatus including one reciprocating gun where the spray gun is reciprocating along a line normal to and spaced from the path of movement of the articles and the spray pattern emanating from the gun is not of a sufliciently large dimension to cover the entire article surface with one sweep of the reciprocating gun. Cylindrical-shaped articles such as hot water heater jackets and washing machine housings are examples of such articles. When I refer to articles of relatively large volume herein, I not only include other large-volume articles than the ones specifically referred to above, but I also include smaller volume articles that are arranged in groups on article holders and each such group effectively is an article of large volume. One type of nonuniform coating that is often obtained on cylindrical surfaces is referred to as barber poling, which is a type of nonuniform coating that consists of alternate, interspersed light and heavy helical bands.

In order to overcome undesirable nonuniform coating, apparatus has been resorted to that incorporates a plurality of spray guns. However, the disadvantages of such apparatus are that it is ditficult to maintain proper adjustment between spray guns, and to obtain proper blending between patterns, the changing of pattern heights is cumbersome and complex, more maintenance service is required and, where conventional compressed air spray guns are utilized, a certain amount of nonuniforrnity may still exist as a result of the erratic sprays that emanate from such spray guns.

Accordingly, it is an object of the present invention to provide an improved method and apparatus for coating articles of a large volume which method and apparatus overcome the objections heretofore noted. It is another object of the present invention to provide a method and apparatus for efficiently coating articles of large volume with a desirable coating in which the movements of the spray source and the articles are coordinated to improve the distribution of the coating material on the articles.

In brief, the above objects are attained in accordance with the present invention by providing a method of and apparatus for spray coating articles in which the articles are bodily moved along a predetermined path in a coating zone. The predetermined path may be in the form of a circle, partial circle, or a loop. As the articles are bodily moving over the predetermined path, each article is rotated about an axis arranged generally perpendicular to the path of article movement. A single-fluid spray source, preferably a rotating device having an annular edge to which liquid is supplied in the form of a Uniform film, is provided in spaced relation to the articles moving over the predetermined path and exteriorly of such articles. In the case where the predetermined path is a circle or loop the source of spray may be located within the circle or loop and project spray particles outwardly from the source in the form of an annular spray for deposition upon the articles. One advantage of using a single-fluid atomizing device is that the coating is carried out in a quiescent atmosphere which results in extremely great transfer efficiency of the spray to the articles being coated. An electrostatic field is desirably established over the surface of the articles being coated as by connecting the opposite terminals of a high voltage source to the articles and the spray source, respectively, for electrostatically depositing or aiding in depositing of the projected spray particles on the articles. The spray source is reciprocated during the coating operation generally parallel to the axes of article rotation over a stroke approximately equal to the height of the articles. Where the articles being coated have both bottom and top surfaces to be coated it is desirable to slightly extend the stroke of the spray source where it is desired to coat these bottom and top surfaces. The reciprocation of the spray source is controlled in such a way that the rotation and the translation of the article and the reciprocation of the spray source are coordinated and controlled so that equal surface areas of a concentric cylindrical surface surrounding the article and rotating therewith would be exposed to approximately equal quantities of spray particles.

In obtaining such coating material distribution, good results have been obtained where cylindrical-shaped articles are rotated in the coating zone a greater or lesser number of revolutions than the number of cycles of reciprocation of the spray source, but in any case the smaller movement is at least a small integral number. The minimum acceptable number of paths traced by the spray on the article depends on the height and circumference dimension of the static pattern emanating from the spray source. By static pattern is meant the deposited pattern made by the spray emanating from the spray source on the target when both the spray source and the target remain stationary in the sense that they are not bodily moved relative to each other. 'I have found in one instance, for example, that where the possibility of synchronizing is not precluded by using positive drives, and where articles no larger than two or three feet in any dimension are being coated with an electrostatic spray device producing a relatively large static pattern, that where the number of cycles of revolution of the articles in the coating zone is greater than the number of cycles of reciprocation of the spray source in the coating zone that the ratio of the revolutions to the reciprocations in the coating zone should desirably be more than four and that where the reciprocations of the spray source in the coating zone are greater than the rotations of the articles in the coating zone then the ratio of the reciprocations to the revolutions in the coating zone should desirably be greater than two.

Satisfactory coating material distribution may also be obtained where the rotation of the article and the reciprocation of the spray source are coordinated and controlled so that each of the rates of article-rotation and sourcereciprocation is different from the other and from any integral multiple of the other. Also satisfactory coating material distribution may be obtained where the rotation of the article and the reciprocation of the spray source are coordinated and controlled so that either the article will have different orientations at successive passages of the spray source through any given point in its cycle of reciprocation or the spray source will be in different points in its cycle at successive passages of the article through any given position of orientation.

The novel features believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof will best be understood by referring to the specification taken in connection with the accompanying drawings in which:

Fig. 1 is a plan view of apparatus illustrating one embodiment of the invention;

Fig. 2 is a side view of the apparatus shown in Fig. 1; and

Fig. 3 is an isometric view of apparatus illustrating another embodiment of the invention.

Referring more particularly to Figs. 1 and 2 of the drawings, the articles 10, illustrated as cylindrical water heater jackets, are mounted on article supports or hooks 12. These supports are connected to conveyor 14 for movement therewith in the directions of arrows 16 over a predetermined path including a loop or circular portion 18. The conveyor 14 is adapted to be driven by a con veyor drive 20 suitably supported above the conveyor. This drive may be varied manually if desired.

As each of the articles is bodily moved around the loop 18 it is positively rotated about its support-axis. The mechanism for rotating the articles comprises gears or sprockets 22, one of which is mounted on each article support, chain 24, chain supporting plate 26, and transmissions 28 and 29 connected by a shaft 30. The transmissions 28 and 29, either or both of which may be variable, are connected to transmit power from the conveyor drive 20 to the plate 26. The chain 24 is mounted on the periphery of plate 26 and moves therewith. As the articles are moved into the loop 18, the gears 22 mounted on each article support positively engage the chain 24 to positively rotate the articles as they move around the loop. By supplying a common drive for the conveyor-movement and article-rotation there is a direct relationship between the speed of bodily movement of the articles by the conveyor and the rate of rotation of the articles. However, even though this is desirable for some coating operations, it is not essential to my invention. Often it is desirable to initiate the rotation of the articles before they move into the loop. In such cases the apparatus shown in Fig. 3 and described hereafter may be used or a friction bar and pulley assembly, for example, as shown in US. Patent No. 2,442,986 may be incorporated in this apparatus to accomplish this purpose.

Disposed centrally of the loop 18 is a spray device 32 which in this specific embodiment comprises atomizing disc 34, disc rotating motor 36, atomizer supporting shaft 38, and a sleeve 40 in which the shaft 38 is adapted to be reciprocated by any suitable reciprocating mechanism well known in the prior art. Also where desired, means such as that shown in the application of William D. Gauthier and Herschel A. Williams, Serial No. 433,514, filed June 1, 1954, may be provided for adjusting the stroke of reciprocation so that the apparatus is adapted for coating articles of varying heights. The disc 34 is maintained at an electrical potential; for example 90 kv., by being connected to one terminal of high voltage source 42 by means of conductor 44-, the other terminal of the high voltage source being grounded as are the articles 10 through the grounding of the conveyor 14. When the disc 34 is maintained at 90 kv. the spacing between the disc edge and the articles 10 is maintained generally in the range of twelve to eighteen inches. Liquid is fed to the upper face of the disc 34 by means of a central opening therein and flows in the form of a thin film to the outer peripheral edge of the disc from where it is atomized. This central opening is connected with a passageway in the motor shaft and a liquid supply pipe 46. The pipe is connected to a suitable source of coatingv material through a suitable pump or other suitable liquid distributing means. The reciprocation speed and length of stroke of reciprocation of the spray device 32 may be controlled as desired, the length of stroke depending upon the height of the article being coated. Such controls are well known in the prior art and this invention is not directed to such controls per se.

In operation, the articles are bodily moved by the conveyor 14 in the direction of the arrows 16 around the loop 18. As each article enters the loop, the gear 22 on its respective article support 12 moves into engagement with the chain 24, and thereafter, assuming the chain 24 is stationary or moving at a rate slower than the rate of conveyor movement, the articles are rotated in the direction of the arrows 48 as they are bodily moving around the loop. If the chain 24 is moving at a rate faster than the rate of conveyor movement then the articles will rotate in an opposite direction to that indicated by the arrows 48. As the articles are moving around the loop, they are coated with a liquid film made up of relatively uniformly distributed spray particles which emanate from the spray device 32 in the form of an annular spray. This spray is electrostatically deposited on the articles by the electrostatic field established be tween the spray device and the articles. The rotation and translation of the articles and the reciprocation of the disc are coordinated and controlled so that equal cylindrical surface areas are exposed to approximately equal quantities of depositing spray particles, and the articles thus obtain a desirable commercial finish.

Referring now to Fig. 3 of the drawings, articles are mounted on article supports 312 which are moved by the conveyor 314 in the direction of the arrows 316 around a loop 318. The conveyor is driven by a variable transmission drive 320 connected to motor 321 by means of shaft 319. Both the drive 320 and motor 321 are mounted on platform 323 suitably supported above the conveyor. As the articles are moved around the loop 318 they are positively rotated by an assembly which comprises gears or sprockets 322 mounted on each article support, rotator chain 324 mounted on the sprockets 327 and 329 and in channel 325, and variable transmission drives 331 and 333 connected to the sprocket 329 and the shaft 319.

Spray device 332 is disposed centrally of the loop 318 and comprises atomizing disc 334, disc rotating motor 336, atomizer supporting shaft 338, and casing 340 in which the shaft is mounted for reciprocation by a mechanical reciprocating means. This means includes chain 341 mounted on gears 343 and 345, and variable transmission 347 connected to gear 345 and the shaft 319. While as shown in this specific embodiment of apparatus both the rate of reciprocation of the spray device and the rate of rotation of the articles are positively controllable, this is not essential to my invention, as only one of those rates needs to be positively controlled.

The atomizing disc 334 is maintained at an electrical potential of, for example, kv. when the spacing between the articles and disc-edge is maintained generally in the range of twelve to eighteen inches by being connected to one terminal of a high voltage source 342, the other terminal of which is grounded as are the articles by means of grounding the conveyor 314. Liquid is fed to the underside of the atomizing disc 334 through a central opening in the disc which is connected to a passageway through the shaft of motor 336 and liquid supply pipe 346, which in turn is connected through a pump or other suitable liquid distributing means to a suitable source of coating material.

The mode of operation of this apparatus is similar to that of the apparatus illustrated in Figs. 1 and 2 anddescribed above. This apparatus is different from the apparatus illustrated in Figs. 1 and 2 in the manner in which the spray device is supported, and in that the rate of reciprocation of the atomizing disc 334 may be directly coordinated and controllel with the rate of rotation of the articles and the rate of translational movement of ticles by the conveyor..:

the ar- While a disc-type single fluid atomizing device is shown in both embodiments described above, it is understood that my invention is not limited to the use of such atomizing device, but also includes within its scope other types of atomizers. The advantage of single fluid atomizers such as described herein is that the spray is deposited in quiescent rather than turbulent air and thus results in greatly improved paint transfer efiiciencies. Quiescent is here used in the sense that the atmosphere or air in which the charged spray particles move as they near deposition on the work is free from air currents of such velocity and volume as to overcome, for many of the coating-material particles, the electrostatic attraction thereof and cause a substantial proportion of the particles to escape deposition on said work.

Although two specific embodiments of the present invention have been shown and described, it is to be understood that still further modification thereof may be made Without departing from the spirit and scope of the appended claims.

I claim:

1. A method of electrostatically spray coating a plurality of vertically elongated articles, comprising moving the articles continually and successively in spaced relation to each other over a horizontal path in an extended coating zone, rotating each article at a predetermined rate about a vertical axis during its movement in said extended coating zone, spraying electrically charged liquid particles from a source spaced exteriorly of the articles in said extended coating zone, maintaining an electrostatic field between the articles in the coating zone and the spray source for electrostatically depositing the spray on the articles, the spray deposited on an article in the coating zone forming a static pattern having a vertical dimension which is but a fraction of the height of the article, reciprocating the spray source over a vertical path at a linear speed such that successive turns of the helical band formed by the spray on the surface of the rotating article during a single stroke of the spray source in one direction do not overlap to an extent such as will result in a uniform coating of the article, the frequency of reciprocation of the spray source being great enough to insure that each article while in the coating zone will receive spray during at least two strokes of the spray source in the same direction and coordinating the rate of reciprocation of the spray source with the rate of article rotation so that the helical band formed during one such stroke of the spray source will be displaced axially of the spray source from the helical bands formed during other such strokes of the spray source.

2. A method set forth in claim 1 with the addition that the path of movement of the rotating article in the coating zone constitutes an arc of a circle of substantial extent, the path of vertical reciprocation of the spray source substantially coinciding with the center of such arc.

References Cited in the file of this patent UNITED STATES PATENTS 2,345,834 Schweitzer Apr. 4, 1944 2,463,422 Ransburg Mar. 1, 1949 2,559,225 Ransburg July 3, 1951 2,672,121 Peeps Mar. 16, 1954 OTHER REFERENCES Ransburg No. 2 Process, published by Ransburg Electro-Coating Corp. (Indianapolis, Indiana), 1953 (page 9 relied on).

Claims (1)

1. A METHOD OF ELECTROSTICALLY SPRAY COATING A PLURALITY OF VERTICALLY ELONGATED ARTICLES, COMPRISING MOVING THE ARTICLES CONTINUALLY AND SUCCESSIVELY IN SPACED RELATION TO EACH OTHER OVER A HORIZONTAL PATH IN AN EXTENDED COATING ZONE, ROTATING EACH ARTICLE AT A PREDETERMINED RATE ABOUT A VERTICAL AXIS DURING AT A PREDETERMINED EXTENDED COATING ZONE, SPRAYING ELECTRICALLY CHARGED LIQUID PARTICLES FROM A SOURCE SPACED EXTERIORLY OF THE ARTICLES IN SAID EXTENDED COATING ZONE, MAINTAINING AN ELECTROSTATIC FIELD BETWEEN THE ARTICLES IN THE COATING ZONE AND THE SPRAY SOURCE FOR ELECTROSTATICALLY DEPOSITING THE SPRAY ON THE ARTICLES, THE SPRAY DEPOSITED ON AN ARTICLE IN THE COATING ZONE FORMING A STATIC PATTERN HAVING A VERTIVAL DIMENSION WHICH IS ABOUT A FRACTION OF THE HEIGHT OF THE ARTICLE, RECIPROCATING THE SPRAY SOURCE OVER A VERTICAL PATH AT A LINEAR SPEED SUCH THAT SUCCESSIVE TURNS OV THE HELICAL BAND FORMED BY THE SPRAY ON THE SURFACE OF THE ROTATING ARTICLE DURING A SINGLE STROKE OF THE SPRAY SOURCE IN ONE DIRECTION DO NOT OVERLAP TO AN EXTENT SUCH AS WILL RESULT IN A UNIFORM COATING OF THE ARTICLE, THE FREQUENCY OF RECIPROCATION OF THE SPRAY SOURCE BEING GREAT ENOUGH TO INSURE THAT EACH ARTICLE WHILE IN THE COATING ZONE WILL RECEIVE SPRAY DURING AT LEAST TWO STROKES OF THE SPRAY SOURCE IN THE SAME DIRECTION AND COORDINATING THE RATE OF RECIPROCATION OF THE SPRAY SOURCE WITH THE RATE OF ARTICLE ROTATION SO THAT THE HELICAL BAND FORMED DURING ONE SUCH STROKE OF THE SPRAY SOURCE WILL BE DISPLACED AXIALLY OF THE SPRAY SOURCE FROM THE HELICAL BANDS FORMES DURING OTHER SUCH STROKES OF THE SPRAY SOURCE

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