US2465128A - Method and apparatus for electrostatic coating - Google Patents

Description

IN V EN TOR. 1 2,, 1i inane/r21; BY M 9 I Patented Mar. 22, 1949 METHOD AND APPARATUS FOR ELECTROSTATIC COATING William A. Starkey, Zionovflie,

assigmn to Bansb :esne Indiana Ind, a-ignor, by urglflectro-Ooatn .,lndianapolis,lnd.,acorporaiionol Application October 31, 1945, Serial No. 825,867 8 Claims. 117-83) This invention relates to the coating of articles, and particularly to improvements in the utilization of electrostatic iorces in the coating of articles. Generally, there have been two diiierent methods for coating articles with the aid oi an electrostatic field. In the first of these methods. exemplified in U. 8. Patent No. 2,334,648, an electrostatic field is employed to cause finely divided particles of coating material to move to and become deposited upon the article to be coated: while in the second method, shown in U. 8. Patent No. 2,359,476, the article is first treated, as by dipping, with an excess of coating material and the electrostatic field is employed as a "detecting" agent to remove such excess. These two methods diiier radically in that in the first method the field acts to cause particles of coating material to move toward the article, while in the second method the field functions to produce particle movement in the diametrically opposite direction, or away from the article.

Determining which oi the two methods above mentioned to employ in any given case usually requires a consideration oi several factors one of which is frequently the shape of the article to be coated. Generally, it is desired to obtain a coating as uniform in thickness as possible; and some articles are so shaped that the electrostatic deposition of coating material will result in greater uniformity oi coating thickness than will the dip-and-detear method, while with articles of other shapes the reverse may be the case. For example, many articles possessing recesses or depressions, may be coated more uniformly by the dip-and-detear method than by the method of electrostatic deposition. On the other hand, as in the case or articles ng extended surfaces free from recesses or projections. or in the ease of articles having relatively sharp edges or corners from which the coating material tends to draw away, it is frequently possible to obtain greater uniformity oi coating thickness by electrostatic deposition than by applying an excess of coating material and subsequently detearing electrostatically. In the case of still other articles. it is diiiicult to obtain a coating of satisfactory uniformity by either method.

My invention is concerned especially with articles oi the last type mentioned, it is my primary and to increase the uniformity oi coating thickness. More specifically, it is my object to obtain the benefits oi the action of an electrostatic field in the coating oi articles which, because of their shape. have been coated onlv with diillculty by the use of an electrostatic field in any manner heretofore proposed. A further object o! my invention is to obtain the economy of operation which characterizes the dip-and-detear method while at the same time securing a greater unitormity of coating thickness than has previously been attainable by the method.

In carrying out my invention, I perform electrostatic deposition and electrostatic detearlng simultaneously, employing an electrostatic field both to remove coating material over portions oi an article coated with an excess of coating material and to propel coating material in finely divided condition towards other portions of the article. More specifically, I apply an excess 0! coating material to those portions of the article best adapted to coating by the dlp-and-detear method, and I then support the article in an electrostatic field into which additional coating material in finely divided condition is introduced. The field acts simultaneously to detear excess coating material from the initially coated portion of the article and to precipitate the finely divided coating material upon the initially bare portions of the article as well as upon any previously coated sharp-edge portions from which the coatlug tends to draw away in the ordinary detearing operation. Proper construction 01' the electrode system used in creating the field will facilitate the simultaneous detearing and deposit oi coating material. In one embodiment of my invention, I employ liquid coating material deteared from one portion of the article to coat electrostatically another portion of the article.

The accompanying drawing illustrates my invention: Fig. l is a diagrammatic side elevation oi a complete coating apparatus; Fig. 2 is an isometric view of a suitable form 01' electrode system; and Fig. 3 is an end elevation of a modified form of apparatus.

The articles l0 illustrated in the drawing as being coated by the apparatus are bombs having an elongated cylindrical body Illa terminating at one end in a rounded nose and provided at the object to iacilitate the coating oi such articles other end with tail fins illb extending radially and axially. Such a bomb will serve as an example of articles of the type to which the present invention is especially applicable, as the application to it of a uniform coating by a method involving either electrostatic detearing or electrostatic deposition alone presents difilculty. In coating the bomb by ordinary electrostatic deposition methods there is a tendency for the coating material to be less heavily deposited on the inner or root portions 01' the fins and on those portions of the body lying adJacent the fin-roots than on other portions of the bomb. If the bomb were completely coated with an excess of coating material and then subjected to an electrostatic detear'ing operation, a relatively long time would be required for the excess to flow down the elongated body to points at which detearing would occur, and there would be a tendency for the coating material to draw away from the edges of the fins at the upper portions thereof. The bomb, however, is well suited for the application of the present invention.

In employing the present invention, I provide a conveyor H from which the bombs ill are adapted to be hung by hangers i2 oi electrical conducting material. The conveyor Ii conveys the bombs over a dip tank l3, and has a depressed portion above such dip tank so that the fins of the bomb will be immersed in and removed from a bath of coating material maintained in the tank. From the dip tank, the conveyor ll extends over a drain board It and into and through a zone in which the bombs are subjected to the action of an electrostatic field maintained between them and an electrode which, in the specific form shown, has three sections-namely, a pair of vertically disposed sections l5 located on opposite sides of the path of bomb travel and a horizontal section l disposed beneath the bombs,

A substantial electrical potential difference is maintained between the bombs Ill and the electrode-sections l and i0.

Conveniently, the electrode-sections i5 and it are electrically interconnected with each other and connected as a unit to one terminal of a high-voltage source H, the other terminal of which is grounded as indicated at It. The conveyor II is grounded, as indicated at 20, as are also the bombs i0 which are electrically connected to the conveyor through the conducting hangers l2. Desirably, the high-voltage source il provides a unidirectional current and serves to maintain between the bombs and the electrode an electrostatic field having an average potential gradient of the order of 10,000 volts per inch.

Mounted in position to discharge between at least one of the vertical electrode sections it and the bombs passing in association therewith are one or more spray guns 22 adapted to discharge into the field a supply of liquid coating material in finely divided condition. To promote uniformity in the coating of the body and nose of the bomb, each of the hangers I: may be rotatabiy supported from the conveyor H and provided with an integral spinner 23 adapted to bear on a stationary spinner rail 24 to rotate the hangers and bombs as they pass through association with the electrode.

In the operation of the apparatus above described, the fins of the bomb are coated with an excess of coating material as a result of their passage through the dip tank 13. As the bombs enter into association with the electrode, the electrostatic field existing over the fins, and especially at the lower edges of the fins, causes excess coating material thereon to be repelled from the fins and attracted toward the electrode. At the same time. the finely divided coating material discharged into the field from the guns 2! becomes charged and attracted to the cylindrical body and nose portions of the bomb. I have found that the eifectiveness of the electrostatic field in producing movement of coating-material particles toward the initially unooated surfaceportions of the bomb and away from the initially coated surface portions can be increased by proper construction of the electrode system. Specifically, it those sections of the electrode opposite the initially uncoated portions of the article-l. e., in this case the electrode-sections lS-embody a limited number of fine wires I! or are otherwise constructed to present sharply curved surfaces toward the article, the transfer 0! coating material to the article will be promoted. 0n the other hand, if the electrode section I. opposite the initially coated portion of the article is formed to present toward the article relatively extended surfaces, the transfer of excess coating material away from the initially coated portion of the article will be furthered. However, when the detearing section of the electrode is located directly beneath the initially coated surface-portions of the article as is the section II in the structure of Figs. 1 and 2, the character oi'tbe surface or surfaces it presents toward the article is relatively unimportant.

If the electrode is constructed in the preferred manner just described, the potential gradient in that portion of the field lying between the lower end of the bomb and the detearing electrodesection it will be greatest adjacent the lower edges of the fins 10b; while in the field-portions, between the bomb and the electrode-sections II, the greatest potential gradient will exist adjacent the fine wires 15'. Desirably, the potential gradient of the field adjacent the wires II is great enough to produce ionization in the regions surrounding such wires in order that coatingmaterial particles entering such regions will become charged similarly to the electrode and will be repelled therefrom and attracted toward the bomb.

In the modified form of the invention illustrated in Fig. 3, I provide an air duct SI arranged to discharge upwardly beneath the bottom, or detearing, electrode-section it. In this instance, the electrode section it disposed beneath the articles, instead of being formed to present relatively extended surfaces toward the article. is preferably constructed, like the sections IS, with a limited number of fine wires around which ionizing zones exist.

The particles of coating material deteared from the lower portion of the article will initially possess charges similar in sign to that of the article, and hence will be repelled therefrom and attracted toward the electrode. However, as they enter the ionizing zones surrounding the wires embodied in the electrode their charges will be reversed, and they will be urged away from the electrode and toward the article. Hie reionlaed particles will hence move upwardly under the joint influence of the field and the upward current of air from the duct 10. While some of the upwardly moving particles will be redeposited on the initially coated lower portion of the article, many will be carried past it and deposited on the initially bare upper portion. While the redeposition of some coating material on the lower portion of the article may prolong the operation,

produce a coating oi satischaracter. Adjacent sharp the tendency to blue is espepronounced. the potential gradient of the ordinarily be relatively steep; and, propotentlal gradient is not sumionising none. the coatwiil be depodted mostly sharp edges. By thus providing deposition of coating material more or selectively upon sharp e ges which tend to blue in the practice of the dip-and-detear proces, my invention overcomes the weaknesses of that coating method.

Twohcmrespectivelytheadhesionofthe coating material to the article-smiaoe and variations in the potential gradient at diiierent points intheileld,contributetothesucceasotsimuiges EEE g s E s particles until after they have impinged upon and become adhered to the suriace. It the detearing electrode-section is provided with relatively extended surfaces. the potential gradient adiacent such surfaces will be relatively slight. and deteared particles of coating material will strike and adhere to the electrode before acquiring a charge similar in sign thereto.

In Figs. 1 and 2 I have shown the detearing electrode-section it as being longer than the electrode-sections I8 and as extending closer to the dip-tank it than do the sections ll. Such an arrangement is indicated when the time required to detear the initially coated portion of the article exceeds that required to cost the initially uncoated portion. When .such a situation exists, it is usually Preferable that the electrostatic detearing operation be started before the deposition operations in order that both may be terminated at approximately the same time.

I claim as my invention:

1. In a method oi applying a liquid coating to an article, the steps of applying an excess of liquid coating material to the bottom portion only thereof, electrostatically removing excess coating material from the bottom portion of the article by subjecting such bottom portion to the action of an electrostatic held of detearing character creating in such ileld an upwardly directed air stream carrying finely divided deteared coating material upwardly toward the uncoated portion of the article, and causing such finely divided coating material to be electrostatically deposited on the initially uncoated upper portion of the article by maintaining an electrostatic field of ionizing character thereover.

2. In apparatus for applying a liquid coating to an article. a support for the article, means for wplying an excess of liquid coating material to the bottom portion of the article, an electrode disposed in spaced relation to a partially coated article on said support, said electrode having a detearing section disposed below the article and one or more discharge sections disposed at the sides of the article, means including a source of high-voltage electricity for maintaining an electrostatic field between the electrode and article, and means for introducing finely divided particles 01 coating material in such held between the article and the discharge electrode-sections.

3. The invention set forth in claim 2 with the addition that said discharge electrode-section has narrow. sharply curved surfaces presented toward an article on said support, said detearing electrode-section having relatively large and flat surfaces presented toward such article.

4. In apparatus for applying a liquid coating to an article, means for applying an excess oi coating material to the lower portion of the article, an electrode, a conveyor for conveying said article successively through association with said coating means and electrode. means including a source of nigh-voltage electricity for maintaining an electrostatic field between the electrode and the article passing in association with it, said electrode comprising a detearing section disposed below the path of article travel and a discharge section disposed at one side of such path, and means for introducing finely divided coating material in the electrostatic field between the article and discharge electrode section.

5. In a method of applying a liquid coating to an article, the steps of apply n an excess of liquid coating material to a portion only 01' the article,

7 creatlngoverhoththeeoatedandunooatedportime o! the that portion a the held over the uncoated portlon having its greatest potential eradlent remote therefrom and that portion of the field over the coat!!! portlon havlnz lta least potentlal eradlent remote theretrom, and lntroduoins finely divided coatln: material into those portions of the field adjacent the allocated portion of the article.

6. Apparamasoetforthlnelalm twlththe addition that said detearlng electrode-section extends closer to the applying means than does said e-aeetlon.

discharge eleetrod wnuan A. STARKIY.

arm on elmtlltlc held with a REFERENCES CITED The following references are of record 111 the file of this patent:

UNITED STATES PATENTS Number Name Date 1,855,869 Push Apr. 26, 1932 2,247,983 Ransbure et al. n July 1, 1943 2,334,648 Ranaburg et al. ---3-- Nov. 16. 1943 2,359,416 Gmvley Oct. 3, 1944

Images