US2421787A - Electrostatic coating method - Google Patents

Description

June .0, 1947. HELMUTH 2,421,787

ELECTROSTATIC COATING METHOD Filed Jan. 26, 1945 2 Sheets-Sheet 1 INVENTOR. M Ema/e0 f, #42 NUT/7.

l June 10,1947. R. E. HELMUTH ELECTROSTATIC COATING METHOD Filed Jan. 26, 1945 2 Sheets-Sheet 2 Patented June 10,1947

. 7 2,421,787 ELECTROSTATIC COATING METHOD Richard E. Helmuth, Indianapolis, Ind., assignor to Harper J. Ransbur Ind., a copartnership g Company, Indianapolis,

Application January 26, 1945, Serial No. 574,722

1 This invention relates to a method and apparatus for coating articles of conducting material by the electrostatic liquid spray process, wherein an electrostatic field of high potential is established between the article being coated acting as a collecting electrode, and a spaced discharge electrode.

In such apparatus, the discharge electrode, being connected to a suitable source of high voltage; is mounted in spaced relation to the article to be coated, which is connected to said source through ground so that it becomes the collecting electrode. Thus, an electrostatic field is established therebetween when the article to be coated is carried by a conveyor into the coating position. Coating material such as paint or lacquer sprayed into such an electrostatic field becomes ionized by action of the discharge electrode and is repelled from it, with the result that the surface of the collecting electrode becomes coated with the material in a manner which is highly efflcient.

In this operation the electrode system must perform two distinct functions in order that the operation proceed to completion in the most satisfactory manner. On the one hand, the discharge electrode must ionize the particles of coating material effectively, and on the other hand it must be responsible for establishing the electrostatic field which eventually accomplishes the deposition of the material upon the surface to be coated. This, means that the discharge electrode must be so constructed that over its area, in general, there will exist portions of very small area about which there will be quite high electrical field strength. This requirement must be met because it is the presence of such small areas that account for the ion production in the air that eventually, by a process of collision, causes the atomized or finely divided coating material sprayed into the field to become ionized.

Coincident with this ionizing function the discharge electrode must establish the field which gives rise to the electrostatic depositing force. To provide for uniform and smoothly brushed out deposition of a liquid coating material, this electrical force must combine with any mecanical action of the spray gun in a manner so that the resultant force is substantially uniform over the surface to be coated. Any type of electrode system may accomplish these desired functions in part, but maximum realization thereof'is best obtainable by the method and apparatus herein disclosed. a

To obtain successful and most efiicient ionization of the liquid coating material at the voltages used, namely 85,000 to 100,000 volts, the discharge 6 Claims. (Cl. 117-93) electrode must comprise conductors of quite small radius sufilciently spaced from each other so that the local electrical field strengthwill be high enough to produce the necessary ionization. Thus the discharge electrode of this invention is composed of fine wires which furnish the necessary source of origin of the ionization. These wires have a size of approximately B. 8: S. #30 (.010

inch). Larger wires correspondingly reduce the efficiency of the ionization and finer wires offer no appreciable increase in efiiciency'and are less rugged.

Fine wires are also necessary for this purpose, particularly when the coating is liquid, because only such wires offer ionization and repulsion in substantially all directions perpendicular to their axes. If the electrode is made of elements which do not have this character, the inherently sticky liquid coating material will tend to build up upon part of the surface of the elements and will eventually fgrow over the region of small area to such an extent that the resulting increase in radius and lower field strength will preclude effective ionization. This explains why wires are superior to such other possible elements as: fiat plate with sharp edges.

The spacing between the discharge electrode wires has likewise been found to be critical. Even though fine wires are used they must be kept reasonably apart. The maximum efficiency is obtained when the wires are separated from each other by a distance which is slightly less than the distance between the discharge electrode and the surface to be coated. At the voltages used this distance betweenwires should be approximately eight inches. The reason therefor is that the field strength about a conductor depends inversely upon its area, and two adjacent conductors at the same potential tend to establish a common field. When said wires'are placed too close together they act as one large wire. For this reason it is also impractical to run an additional set of cross wires perpendicular thereto, since the aggregate under these conditions similates a continuous conducting sheet and low ionization efiiciency results.

In addition to the ionization function of the discharge electrode, it must also establish the electrostatic field which precipitates the ch'arged particles on to the article. The nature of this field determines the distribution of the precipitated coating. In the stronger portions of the field the liquid coating will be applied heavier, while in the weaker portions of th'e field it will be lighter. It has been the practice to describe these electric fields in terms of the so-called elecfields by fewer lines. The finely divided particles of coating material sprayed as a liquid, which are" ionized and free to move in such field can in general be thought to move along these lines of force. The distribution of such lines of force may be altered by altering the nature of the discharge electrode, or by changing its position with respect to the article. Since the electrode is presented herein as a series of fine wire elements, the field will naturally originate at these wires and hence the shape and distribution of the field will depend in a large degree upon the arrangement and distribution of these wires.

utilizing the brush-like character of the field distribution about such fine wires to insure that.

the depositing forces created thereby will uniformly sweep over each portion of the collecting surface as this surfacelmoves into and'becomes one terminal of the lines constituting this field. This is accomplished by arranging the discharge electrode wires transversely of each path of movement to direct an infinite number of axially symmetric linesof force in a corresponding infinite number of planes extending parallel to or in the direction of the path of travel of the article.

Thus an infinite number of axially symmetric lines of force, which terminate on the surface of the article as the collecting electrode, are produced from and along the common axis of the 'wires' and since the article is moving relative thereto, successive portions thereof become in turn the terminating surface of the lines of force such as to effect a smooth brushing action. Further, by reason of the relative arrangement of the discharge electrode and article in respect to itspath of travel, the fine wires as the origin of the axially symmetric lines of force will be of such radius as to ionize and repel substantially all of the coating particles which are found in a right cylinder coaxial therewith.

wherein the ionization effected by a single wire electrode may flicker back and forth along its length and thereby, have a tendency to vary the uniformity of the resulting coating, it is of substantial benefit to compensate for this action through providing a series of such wires arranged in parallel spaced relation, whereby any tendency of unevenness in the liquid coating deposi tion resulting from a single wire will be ofiset and corrected by the successive depositions resulting from the lines of force originating with the, succeeding several parallel wires.

However, as the number of electrode wires-is I increased the efiectiveareaz of the electrode struc--.

ture is increased, and so the. electrical capacitance of'the system composed offelectrode structure and surface. to be coatedpoifl, collecting electrode, is

control resides in the fact that the high voltage source is so constructed that only a very small electrical current can flow from it and thus the energy released in event of accidental spark is small and so relatively non-hazardous.

However, if the capacitance of the system composed of the discharge electrode and collecting electrode is large, energy can be stored therein which in event of accidental discharge can deliver a relatively high current spark and thus destroy the safety control normally vested in the voltage supply. It is important therefore to keep this capacity small when spraying inflammable- Ifthe electrode structure is of some material. size, its capacity is kept small by making its spacing from the coated surfaces as large as is practical. All electrically insulating materials which come between the discharge electrode and collecting electrode tend to increase this mutual capacity by virtue of the dielectric action of such materials. It is important therefore to avoid having any insulating material between the discharge electrodes and the collecting electrode when the electrodes are of appreciable area.

The full nature of the invention will be understood from the accompanying drawings and the following description and claims:

Fig. 1 is a'perspective view of an apparatus for carrying out the invention.

Fig. 2 is a schematic illustration thereof indicating the lines of force of the electrostatic field.

Fig. 2a is a view showing a partial section taken transversely of Fig. 2.

Fig. 3 is a schematic illustration similar to Fig. 2 with the lines of force disposed to cause streaking of the coating material.

Fig. 4 is a perspective view of an apparatus as applied to coating'an article having a cylindrical surface with its axis extending in its path of travel.

Fig. 5 is a schematic illustration of Fig. 4 indieating the lines of force of the electrostatic field.

Fig. 6 is the same as Fig. 5 showing the lines of force disposed to cause streakingof the coating material.

Fig. 7 is a perspective view of an apparatus as applied to a series of cylindrical articles to be coated revolving about an. axis perpendicular to their path of travel.

For illustrating the invention there is shown in Fig. 1 a conveyor track 10 supporting a moving conveyor chain ll driven by suitable means, not shown herein, to convey a series of articles to be coated, in the form of metal panels I3 supported in alignment by the hooks l4 carried by the conveyor or chain II, said articles to be conveyed slowly in the direction indicated by the arrows.

Mounted to one side of the path of travel of the article there is a discharge electrode comprising the insulated supporting legs I5 carrying a frame l6 having its leading and trailing members I! out of the general plane of the electrode system. Carried by the frame I6 there are a series of fine ionizing wires [8 arranged in parallel spaced relation to'each other and of such length as to extend aboveand below the surface of the article l3. Saidwire's comprise the discharge electrode,

. beingconnected:to,-'one terminal-of a source of increased. This fact inustbe-seriously considered when this general method is used to apply infiam-f mable materials to relatively'large surfaces. In such electrostaticarrangements the major safety high voltagef l lfi'ihe opposite terminal of said source gzf high voltage is grounded, as is the track In. through ground, the articles l3 are connected with the other terminal of the source of high voltage by reason of the conductivity of the conveyor chain and the supporting hooks it. said articles comprise a collecting electrode at high potential difference with respect to the discharge electrode wires l8. Between the electrode wires 3 and the articles to be coated, there is established an electrostatic field of high potential.

The coating material in the form of a liquid carrying finely divided particles is sprayed into the electrostatic field. As shown herein, the coating material is sprayed into the field by a series of spray guns supported upon a standard 2|. Upon being sprayed into the field the coating particles become ionized and are then attracted to the collecting electrode to be coated, as well as repelled by the discharge electrode wires l8 for electrostatic deposition. For this purpose the discharge electrode wires l8 are preferably equally spaced from each other and at substantially twice the sparking distance from the surface to be coated.

Each of the electrode Wires act as a separate and independent electrode of such small area as to develop the maximum ionization to thereby effectively ionize and repel substantially all coating particles which are found within a space defined by a right cylinder coaxially situated in respect to each of said wires. At the voltage generally employed, namely 85,000 to 100,000 volts, said wires should be spaced about eight inches from each other and at about twice the spark over distance from the surface of the article as the said cylindrical space or field of effective ionization thereabout ismost effectively established under these conditions.

As best indicated schematically in Fig. 2, each of said wires l8 generates, along their axial length, an infinite number of electrical lines of force indicated at 22. These lines of force terminate with collecting electrode l3 along the surface to be coated. Near the boundary of said surface they will terminate in part on the edge of the collecting electrode and in part on the reverse side thereof with diminishing intensity from its edge as shown at 24. Fig. 2a shows a detailed 'view of the nature of these lines near the boundary. The lines of force thus generated extend and lie in an infinite number of planes, indicated schematically at 23, which are perpendicular to the wires l8 and the surface of the collecting electrode. The discharge electrodes 18 are arranged transversely of the path of travel of the collecting electrode, so that the infinite number of planes of the lines of force extend parallel to or in the direction of travel of the article.

Thus, as the article is caused to move past the discharge electrode, its surface to be coated will be evenly swept or brushed in its direction of travel and its edges will be brushed with a field of diminishing intensity as the lines of force progress over the edges and onto the reverse side, terminating thereon. The ionized particles of coating material being directedby the lines of force will therefore be deposited upon the surface to be coated by such brushing action. The edges will receive a, corresponding coating and the reverse side due to electrostatic wrap-around will be coated with a film that feathers out away from the edges. Both the edge coating and reverse side coating are very desirable features which can only be obtained in conjunction with the uniform surface coating by using this method. This action may be likened to that of the fiat side of a long paint brush being drawn across the surface in its direction of travel, the lines of force corresponding to the bristles of the brush end of the collecting electrode. Wherein there 6 are a series of electrode wires spaced in parallel relation transversely of the direction of travel, such electrode system ha the effect of a series of brushes. 1

The effect of this arrangement is such that the liquid coating material deposited electrostatically upon the surface will be smoothed out from one side to the other over the entire surface of the article facing the electrode and aroundits edges. This results in not only an efllcient application of the coating material, but avoids streaking and produces a uniformly smooth coating.

For example, and by way of comparison, there is schematically shown in Fig. 3 an arrangement wherein the wire electrodesllla are arranged longitudinally of the direction of movement of the article I3, and the lines of force 22a generated thereby extend in an infinite number of planes transversely of, or across the path of movement of the article. 'The effect of this arrangement is to leave a streaked coating on the surface, the streaks running in the direction of its path of travel by reason of variation in the strength of the lines of force crosswise of the surface.

As the strength of the lines of force taken longitudinally of the discharge electrode wires all lie in the same longitudinal plane with respect to the direction of travel, this streaking effect will be accentuated as the article progresses past the discharge electrode. This is an undesirable arrangement of the lines of force resulting in I streaking of such surfaces as are of substantial area, and which it is the purpose of this invention to avoid.

In Fig. 4 the pedestals I I0 support in electrical contact for continuous movement thereover, by the rollers III, the article H3 in the form of a cylindrical pipe having substantial length.

Suitable means, not shown, is provided for continuously moving the pipe over said supports and through the coating zone. The discharge electrode of this zone is suspended by the insulated rods H5 which carry the frame H6. The frame in turn carries a series of equally spaced discharge electrode wires H8, which extend completely about the pipe H3 in a series of spaced parallel planes transversely arranged relative to the path of travel of the pipe. The electrode wires H8 are electrically connected to one terminal of the high voltage source H9, the other terminal of which is grounded as is the carrier support H0 whereby the pipe H3 constitutes a collecting electrode spaced from the discharge electrode wires H8 at a slightly greater distance than said wires are spaced from each other to create an electrostatic field therebetween as above described. A group of spray guns I20 are arranged to discharge the coating material into the field.

As schematically illustrated in Fig. 5, showing in effect a vertical section through the electrodes, the lines of force indicated at I22 extend inwardly from the surrounding electrode wires to the surface of the pipe in an infinite number of planes extending longitudinally of the direction of travel of the pipe and transversely of the electrode wires H8. These lines of force extend in their respective planes axially symmetrical about the electrode wires-that is, in a symmetrical pattern on each side thereof with the wire constituting the axial center of such pattern. Said axially symmetric lines of force sweep along the surface of the pipe in a manner similar to the fiat side of a paint brush being applied thereto in the direction of its path of traveL- The stronger lines of force are indicated in Fig. at A-A' and the weaker lines of force at 3-3, but since the lines of force emanating from the wires at C and D will augment each other as they approach the line 13-13, the effect will be that their composite forces will even out over the corresponding surface of the article. It is to be noted that in this arrangement the entire space between the electrode system and the coated surface is filled with lines of force which all'terminate on the surface. A uniform deposition of the material on the surface is thus effected and thereby more or less coating material being applied along its surface in the direction of its travel in streaky relation is thus avoided.

By way of comparison, Fig. 6 represents a similar schematic relation of the electrodes relative to the article wherein the discharge electrode wires il8a are arranged about the pipe H3 to extend longitudinally of its direction of movement. With this arrangement the stronger lines of force will exist at XX. But the field at the surface of the article between points X will vary due to the increasing and decreasing strength of the lines of force applied to this portion of the surface. Since the infinite number of planes in which the lines of force X-X and X-Y' lie are alltransverse of the path of movement, the space between the electrode wires and the surface will not be completely filled with lines of force which grounded. They are individually carried by the.

conveyor chain through their respective hangers 2M and are caused to continuously rotate about. a

their axial center through theroller and shoe engagement 2l2, The discharge'electrodeis sup ported upon the insulated legs 2I5' by a frame 2l6 having outwardly flared ends 2|! to support a series of spaced discharge electrode wires 218 positioned on each side of the articles and extending above and below them transversely of bottom surfaces of the collecting electrode. Thus, the characteristics of the invention is in all respects the same in this application thereof shownin Fig. 7 as. above described with respect'to Figs. 1 and 2.

From the foregoing it will be evident that the invention is directed toward the application of a uniform and smoothed or brushed out adhesive coating of liquid material by an electrostatic action particularly desirable in coating articles of conductive material wherein'the surface to be coated is of appreciable area, and wherein the articles are caused to continuously move in the same direction and path of travel past the field or discharge electrode.

' It is immaterial as to whether or not such surface to be coated is substantially flat as in Fig. l; is cylindrical with its axis extending in the direction of travel as in Fig. 4; or is cylindrical and rotated with its axis extending transversely of the direction of travel as in Fig. 7. Thus, the invention includes properly spacing a series of fine discharge electrode wires from each article and each other to give proper effect to their respective radii of ionization so as to ionize and repel substantially all ofthe coating particles introduced'into that portion of the field embraced within an imaginative right cylinder situated coaxially with respect to the axis of the respective wires.

Particularly, the invention involves the relation between the axially symmetric lines of force having their origin in the respective wires, with such wires constituting their axial center, whereby they will lie in an infinite number of parallel planes extending transversely to the axis of their wires, said planes extending longitudinally of the direction of travel of the articles;

or wherein; the article is moved in a plane perpendicular to said planes through the axial symmetric lines of force. It may further be noted that such axially symmetric lines of force terminate on the conductive surface to be coated in such a way that successive portions of said surface become in turn the collecting electrode or terminal of said lines of force as said portions their path of travel in the direction of the ar rows. The electrodes are connected to one .ter-

mirral of the source of voltage iil the other terminal of which is groundedf' The high poten 3 tial electrostatic field is thus established between the electrode system and the article being coated.) A series of spray guns-20 are mounted on astandard 2| to direct the liquid coating material into preciable area. Thus, in this arrangement the effective lines of force are directed by the vertically disposed electrode wires axially symmetric thereto in an infinite number of planes extending longitudially of the path of movement of the article. This similarly applies to the lines of force generated by the wires and terminating due to electrostatic wrap-around" on the top and so y -fThe finvention claimed is:

in turn become positioned opposite the discharge electrode.

The apparatus, as shown in-Fig. 1, 4 and 7, is preferably constructed in such a way that the leading edge of the electrode frame which supports the fine ionizing wires is removed out of the general plane of said wires on the electrode proper. It is so arranged whereby the incoming particles of coating-materialwill first encounter an ionizing element (fine wire) of the electrode system, rather than be intercepted and affected bythe non-ionizingleading edge of the supporti e' ra l The' method which comprises establishing over the surface of an article t'obe' coated an electrostatic field having an elongated region of substantially unlformintensity, thatportion of said field remote from the'articlecomprising an elongated ionizing zone, spraying an adherent liquid coating material in a dispersed state into said field for deposit on said articleQand moving said article transverse to the, direction Iof elonga- :ofliquid coating material, the. distance between '-;face to be coated being materially less than, and v r 5 said elongated ionizing zone and the article-surthe. length of the ionizing zone being greater than,

s 9 thedimension of such surface in a direction parallel to such ionizing zone.

2. The method which comprises establishing over the surface of an article to be coated an electrostatic field having a plurality of parallel, elongated regions of substantially uniform intensity, that portion of said field remote from the article comprising a plurality of substantially parallel, elongated ionizing zones, spraying an adherent liquid coating material in a dispersed state into said field for deposit on said article. and moving said article progressively transverse to the direction of elongation of said regions while introducing said spray of liquid coating material, the distance between said elongated ionizing zones and the article-surface to be coated being materially less than, and the length of the ionizing zones being greater than, the dimension of such surface in a direction parallel to such ionizing zones;

3. The method which comprises establishing over the surface of an article to be coated an electrostatic field having an elongated region of substantially uniform intensity, that portion of said field remote from the article comprising an elongated ionizing zone, spraying an adherent liquid coating material in a dispersed state into said field in a direction transverse to the direction of elongation of said region for deposit on said article, and moving said article transverse to the direction of elongation of said region, the distance between said elongated ionizing zone and the article-surface to be coated being materially less than, and the length of the ionizing zone being greater than, the dimension of such suriace in a direction parallel to such ionizing zone.

4. The method which comprises establishing over the surface of an article to be coated an electrostatic field having a plurality of parallel elongated regions of substantially uniform intensity, that' portion of said field remote from the article comprising a plurality of substantially parallel elongated ionizing zones, spraying an adherent liquid coating material in a dispersed state into said field in a direction transverse to the direction of elongation of said regions for deposit on said article, and moving said article progressively'transverse tothe direction of elongation of said regions, the distance between said elongated ionizing zones and the'article-surface to be coated being materially less than, and the length of the ionizing zones being greater than,

10 the dimension of such surface in a direction par allel to such ionizing zones.

5. The method which comprises establishing over the surface of an article to be coated an electrostatic field having an elongated region of substantially uniform intensity, that portion of said field remote from the article comprising an elongated ionizing zone, spraying an adherent liquid coating material in a dispersed state into said field for deposit on said article, and rotating said article on an axis extending parallel to the direction of elongation of said region while moving said article transverse to the direction of elongation of said region, the distance between said elongated ionizing zone and the article-surface to be coated being materially less than, and the length of the ionizing zone being greater than,

the dimension of such surface in a direction parallel to such ionizing zone.

6. The method which comprises establishing over the surface of an article to be coated an electrostatic field having an elongated region of substantially uniform intensity, that portion of said field remote from the article comprising an elongated ionizing zone, spraying an adherent liquid coating material in a dispersed state into said field transversely to the direction of elongation of said region for deposit on said article, and rotating said article on an axis parallel to the direction of elongation of said region while moving said article transverse to the direction of elongation of said region, the distance between a said elongated ionizing zone and the article-surface to be coated being materially less than, and the length of the ionizing zone being greater than, the dimension of such surface in a direction parallel to such ionizing zone.

RICHARD E. HELMUTH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Date

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