US2425652A - Method and apparatus for spray coating articles - Google Patents

Description

Aug. 12, 1947. w, STARKEY 2,425,652

METHOD AND APPARATUS FOR SPRAY COATING ARTICLES Filed Jan. 22, 1944 PIEi. 1

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METHOD AND APPARATU$ F-OR SPRAY COATING ARTICLES 7 2 Sheets-Sheet 2 Filed Jan. z, 1944 IF?- I23 PIE. 4

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Patented Aug. 32, 1947 METHOD AND APPARATUS FOR SPRAY (JOATING ARTICLES William A. Starkey, Zionsvllle, Ind., assignor to liiarper J. Ransburg Company, Indianapolis,

Ind., a copartnership Application January 22, 1944, Serial No. 519,333

17 Claims. (Cl. 11717) This invention relates to a method and apparatus for spray coating articles in an electrostatic field as more particularly set forth and described in Letters Patent to H. P. Ransburg et al, No. 2,247,963, issued July 1, 1941, for Apparatus for spray coating articles and No. 2,334,648, entitled, "Method of spray coating articles. In apparatus embodying this invention it has heretofore been the practice to provide a collecting electrode and a discharge electrode between which en electrostatic field is set up. Usually the article to be coated is the collecting electrode, either of itself if of conducting material or in association with its support of conducting material. The discharge electrode is usually in the form of a series of conducting wires or a screen spaced from the article or collecting electrode for providing an electrostatic field therebetween. A source of high voltage is connected with said electrodes, either of which may be the ground with the other at a relative highpotential. Genorally, however, for convenience, the article or collecting electrode is connected to ground, whereas the discharge electrode is directly connected with the high voltage source through a conductor so as to constitute the electrode of high potential.

The purpose of this invention is to provide a shielding electrode in association with the article or collecting electrode wherein such article is of such physical shape that a portion thereof may not be satisfactorily coated. It is found that parts of some articles do not receive sufilcient coating when sprayed in an electrostatic field, particularly when such article is shaped to terminate in a point or is formed with a protrusion. This is due to the fact that the electrostatic field strength in the region of such point or protruding surface, is increased to a degree comparable with the field strength about the discharge electrode. Thus such an article, in the region of its reduced prptruding surface becomes an ionizing source and tends to repel the particles of coating initially intended therefor.

Still other portions of the article may receive too much coating material due to the fact that the field strength, and hence the precipitating force, is higher in such area than at adjacent por-' tions'of the article, but still not sufliciently high to produce the reionization effect mentioned above. Thus in this instance it is desirable to attain a balance or equalization of the field strength or a precipitating force over such varying portions.

It is the object of this invention to provide a method and apparatus such as will overcome these difliculties and assur an even and complete coating over such varying portions of the article. This is accomplished by mounting on the support therefor, the article or in association therewith an auxiliary electrode of the same potential as the article or its support, extending in such relation to that portion of the article as to electrically shield such portion and thus reduce the field strength at such location. Such reduction in the field strength prevents, in the one case, the reionization of the coating particles, thus permitting them to be attracted to and reach such portion in the same manner and to the same extent as in respect to the normal surface of the article, and in the other case, maintains a balance or equalization of the field strength over theentire surface to be coated.

The full nature or the invention will be understood irom the accompanying drawings and the following description and claims:

Fig. 1 is a perspective view of an electrostatic spray booth in which articles are spray coated in an electrostatic fleld.

Fig. 2 is an enlarged view of an article to be coated with a shielding electrode associated therewith.

Fig. 3 is a perspective view of a modified article having a protrusion with a shielding electrode carried thereby to maintain balance oi. field strength.

Figure 4 is a vertical section through the article shown in Figure in association with the apparatus.

In the drawings there is illustrated a spray booth Ill having mounted therein a discharge electrode ll directly connected by the lead-in line I2 with a source of high voltage as indicated at l3.

As shown herein there are two spaced discharge electrodes, each supported within the booth by the insulators l4 and connected by the lead line ll. Between said electrodes there is positioned an article I 6 to be coated, so that said article is in the line of spray of a coating material discharged from one or more suitably positioned spray guns ll mounted on a support l8. Said article is grounded through a support l8 to the other terminal of the source of high voltage I3; Thus the field into which the spray coating is discharged by the spray gun is electrostatlcally established between the grounded article and the electrode l I at high potential, This has the effect of ionizing the spray particles and causing them to be attracted toward and precipitated upon said article.

As shown herein there may be a series 01' said articles conveyed through the spray booth upon a suitable conveyor indicated at 20 so that said articles become one electrode of the electrostatic field and thus are successively coated as they pass therethrough. Also, they may be caused-to rotate as they pass through the field wherein it is desired tocoat the surrounding surface, by the provision of a rack and 'pinion arrangement between the booth and the supports I9, such as the pinion 2 I and rack bar 22.

As shown in Figs. 1 and 2, the surface of the article to be coated terminates n a reduced or pointedsurfaoe. For illustration there is shown a projectile windshield. By virtue of the shape of such an article the conical pointed end portion thereof will not ordinarily receive a satisfactory coating due to the fact that the electrostatic field strength in its region is increased to the point where it is comparable with the field strength about the discharge electrode so that this portion becomes an ionizing source tending to repel the coating particles initially intended to coat it. To overcome this condition there is mounted on the support I8 or in association with the article or collecting electrode a shielding electrode. Said shielding electrode is herein indicated at 23 to provide a sharp point 24 suitably spaced from but in field association with the pointed portion of the article as indicated at 25. As the paint particles of the spray leave the gun and enter the electrostatic field they become ionized or electrically charged opposite to the grounded article so as to be normally attracted thereby. However, as they approach the pointed portion of small area they will receive a charge of the same sign as the article and hence will be repelled, leaving such pointed area without coating. This is due to the increased field strength of the grounded article at this point and over the adjacent portion thereof.

To avoid this condition the pointed shielding electrode 23 is mounted to partially electrically shield the pointed portion of the article and thus reduce the field strength at this point. Such reduction in the field strength due to the shielding electrode prevents the reionization of the coating particles, and thus permits them to be similarly attracted to this portion as they are to that portion of greater area.

Whereas for purposes of illustration a particular article has been shown herein havin a conical pointed end, it is not intended to limit the invention to this particular shape or arrangement of shielding electrode, since the invention is applicable to articles of varying shapes wherein portions of their surface are such as to ionize and repel coating particles with which it is to be coated in an electrostatic field.

v Whereas the invention is herein shown as embodying an article of conducting material which with or independently of its support may be grounded or otherwise connected with the source of high voltage to constitute a collecting electrode, it is to be understood that wherein the article itself may be of non-conducting material but supported upon such collecting electrode,-it in effect constitutes a collecting electrode in association therewith, and therefore within the terms of the appended claims such article of non-conducting material with its support constitutes the collecting electrode of one potential in respect to the discharge electrode of a different potential.v

In the modified form of article illustrated in Fig. 3 there is shown a structure which may be a pulley I IS with a protruding or projecting portion suitable co'nveyor through a field which is electrostatically charged between thearticle as the collecting electrode and a spaced discharge electrode. Referring now to Figure 4, the pulley or other article 8 is illustrated as constructed of non-conducting material and mounted upon a support H9. The support H8 is similar to the support I9 of Figure 1 except that at its upper end it carries a flanged member I28 adapted to receive this type of article. Member I26 has an upwardly projecting section I21 that fits loosely within a recess I28 formed on the underside of the article as an incident to the forming of its projecting portion I25.

In mounting the article on the member I26 the lower surface of the article rests upon the fiange of the member and the section I21 extends only partially into the recess I28 with clearance being left between the upper surface of section I21 and the undersurface of projection I25. The parts I26, I21 and I I9 are,'of course, of conducting material. Support H9 is suitably grounded and is associated with a conveyor I20.

Oppositely disposed discharge electrodes III mounted in spaced relation on opposite sides of the article are electrically interconnected by a conductor III and are connected by a conductor II2 to one terminal of a high voltage source N2, the opposite terminal of which is connected to ground.

One or more spray guns 1 are preferably mounted forwardly (toward the reader as viewed in Fig. 4) of the discharge electrodes and are directed so as to project the coating material into the influence of the electrostatic field. The foregoing parts, the electrical connections therebetween, and the general arrangement and purpose correspond in all respects to the corresponding counterparts, connections and purposes shown and described in connection with Figure 1. However, when the article IIB is of non-conducting material as shown, one of the terminals I24 (the left-hand'terminal in Figure 4) of the shielding electrode I23 is of sufllcient length to contact the member I21-I28 and thereby establish electrical connection between the shielding electrode and support H9 which is connected to ground.

When the article III is of conducting material there is, of course, no necessity for lengthening any of the terminals I24, the electrical connection between the shielding electrode and ground being eifected through the conductive material of the,

article. I

The shielding electrode I23, I2 is of the same potential as the article or its support in'the same manner as above described with respect to the electrode 23, 24. In this modification the projecting portion of the article is of such area, as distinguished from that of Figs. 1 and 2, that it will not have the effect or reionizing the spray coating particles such as to repel them as above described. But by reason of the surface area such as will not concentrate the electrostatic forces as in the case of a pointed projection, the closer proximity of such projecting portion of the article to the discharge electrode will be subjected to a higher precipitating force. Its closer proximity to the discharge electrode relative to other portions of the article renders the field strength over such projecting portion higher, and hence tion on said article the precipitating force is proportionately increased. This results in a greater amount of coating being precipitated on such projecting portion by reason of the higher field strength and the precipitating force than occurs on the more remote portions of the article.

To balance and equalize the field strength so that substantially the same coating will be collected on both the projecting portion and the more remote portions of the article, the screening or shielding electrode I23, I24 of the same potential is mounted to project therefrom such as to intercept and weaken the precipitating force of the field to thereby render such portion comparable to the more remote portions.

Thus the phenomena of this condition is such that at one extreme the projecting portion. as in the first instance, is screened to prevent reionization and repulsion of the coating particles, so that it will receive practically no coating, whereas in the second instance, similar screening thereof prevents it from receiving excessive coating. In both instances the projecting portion is screened by a shielding electrode for the purpose of balancing and equalizing the precipitating forces of the field, and, therefore, obtain evenness of coating in respect to the more remote portions of the article.

It is to be understood, of course, that the specific articles illustrated are but examples of a wide variety of articles having three-dimensional surfaces adapted to be coated in the practice of this invention. Whatever the precise shape of the three-dimensional surface may be, my invention can be utilized to produce a localized modification of field strength and thus either to prevent reionization and attract particles to a surface-portion which otherwise would be too thinly coated, as in Figs. 1 and 2, or to cause reionization and repel particles from a surface portion which otherwise would be too thickly coated, as in Fig. 3. By the expression three-dimensional surface, I mean to denote a or not they are separated by well defined edges, do not lie in a common plane. Thus, any surface surface parts of which, whether,

' ing a projecting portion wherein said article conportion of a cube which extends into two or more faces thereof, the conoidal surface of the windshield l6, and the contoured surfaces of the pulley H6 are all three-dimensional.

The invention claimed is:

1. The method of coating an article having a projecting portion, comprising the creation of an electrostatic field induced by spaced electrodes having a high difference of potential, and wherein said article constitutes one of said electrodes, introducing finely divided coating material into said field to :be ionized thereby for electrical deposiand reducing the field strength over and about the projecting portion of said article.

2. In a method of coating in an electrostatic field an article having a projecting portion capable of creating an adjacent ionizing zone when the article is in the field, the steps of creating an electrostatic field over the surface of the article, introducing finely divided coating material into said field to be ionized thereby and electrostatically deposited on said article, and reducing the field strength over the projecting portion of the article to prevent reionization of coating material particles approaching said portion.

3. The method of coating an article having a projecting portion, comprising the creation of an electrostatic field induced by spaced electrodes having a high difierence of potential and wherein stitutes a collecting electrode, a discharge electrode mounted in spaced relation to said article, electrical means producing a. high potential difference between said discharge electrode and article, means for introducing finely divided coating material into the electrostatic field between said article and discharge electrode, and an auxiliary shielding electrode electrically connected to said article having a terminal associated with but spaced from the projecting portion thereof. 5. In an apparatus for coating an .article having a projecting portion wherein said article constitutes a collecting electrode, a discharge electrode mounted in spaced relation to said article,

electrical means producing a. high potential difference between said discharge electrode and article, means for introducing finely divided coating material into the electrostatic field between said article and discharge electrode, said material to be ionized thereby for electrical precipitation over the surface of said article, and means in said field for reducing the field strength about said projecting portion.

6. In an apparatus for coating an article having a projecting portion trode mounted in spaced relation to said article, electrical means producing a high potential difference between said discharge electrode and article, means for introducing finely divided coating material into the electrostatic field between said article and discharge electrode, and an auxiliary shielding electrode of the same potential as said article having a terminal associated with but spaced from a projecting portion thereof.

'7. In an apparatus for coating an article having a projecting portion wherein said article constitutes a collecting electrode, comprising a discharge electrode mounted in spaced relation to said article, electrical means for producing a high potential difference .between said electrode and article to create an electrostatic field therebe- "tween, means for introducing finely divided coating material into said field directed toward said article for electrical attraction and precipitation thereon, and an auxiliary shielding electrode carried with said article and of the same potential, said shielding electrode having a terminal associated with but spaced from the projecting portion of said article.

8. In an apparatus for coating a plurality of articles wherein each of said articles constitutes a collecting electrode having a projecting portion, a conveyor including a series of supports for carrying said articles through a predetermined path, a discharge electrode spaced from the path of movement of said articles, electrical means for producing a high potential difference between said discharge electrode and articles, means for introducing finely divided coating material into the space between said articles and discharge electrode, and an auxiliary shielding electrode carried by and of the same potential as each of said articles having a terminal associated with but spaced from the projecting portion thereof.

9. In an apparatus for coating an article havwherein said article con-., stitutes a collecting electrode, a discharge elecaasaesa ing a projecting portion, a support for said article of conducting material whereby said support and article constitute a, collecting electrode, a discharge electrode mounted in spaced relation to said collecting electrode, electrical means producing an electrostatic field between said electrodes, means for introducing finely divided coating material into said field, and an auxiliary shielding electrode electrically connected to said collecting electrode having a terminal associated with but spaced from the projecting portion of said article. 7

10. In an apparatus for coating an article having a projecting portion, a support for said article of conducting material, said article and article support constituting a collecting electrode, a discharge electrode mounted in spaced relation to said collecting electrode, electrical means producing an electrostatic field between said electrodes, means for introducing finelydivided coating material into said field for electrostatic deposition on said article, and means in said field for reducing the field strength about the projecting portion thereof.

11. In a method of coating a three-dimensional surface of an article, the steps of making the article an electrode in an electrostatic field, introducing finely divided coating material into said field for electrostatic deposition on such surface, and modifying the strength of the field adjacent a portion of said surface by associating with said portion in spaced relation thereto and in intercepting relation with respect to said field an auxiliary conductor maintained at a predetermined potential.

12. In a method of coating a three-dimensional surface of an article, the steps of making the article an electrode in an electrostatic field, introducing finely divided coating material into said field for electrostatic deposition on such surface, and modifying the strength of the field adjacent a portion of said surface by associating with said portion in intercepting relation with respect to said field an auxiliary conductor maintained at a predetermined potential and having a terminal spaced from such surface-portion.

13. In a method of coating a three-dimensional surface of an article, the steps of making the artl cle an electrode in an electrostatic field, introducing finely divided coating material into said field for electrostatic deposition on such surface, and modifying the strength of the field adjacent a portion of said surface by creating within said field and at points spaced from the surface to be coated an electrical potential substantially equal to that of the article.

- support, a high-voltage source for maintainin8 an electrostatic field in the gaseous medium between theelectrode and article, and means for introducing finely divided particles of coatin material into said field to be propelled toward such surface and deposited thereon by the action of the field,- means for modifying the strength of said field over a portion of such surface to produce a localized variation in the particle-depositing effect of the field, said means comprising an auxiliary electrode of conducting material spaced in the field from said three-dimensional surface and maintained at a predetermined potential,

15. In an apparatus for electrostatically coating a surface of an article formed of electrically conducting material, a discharge electrode, means for conveying the article past and in spaced relation to said electrode, means including a highvoltage source for maintaining an electrostatic field between the electrode and the article moving past it, means for introducing finely divided coating material into said field, and means including a member of conducting material in contact with said article for reducing the strength of the electrostatic field over a portion of the surface to be coated.

16. In apparatus for electrostatically coating a surface of an article, a discharge electrode, means for conveying the article past and in spaced relation to said electrode, means including a, highvoltage source for maintaining an electrostatic field between the electrode and the article moving past it, means for introducing finely divided coating material into said field, and means includinga member of conducting material supported by said article for reducing the strength of the electrostatic field over a portion of the surface to be coated.

17. In apparatus for electrostatically coating a a surface of an article, a discharge electrode, an article-support, means for moving said support past said electrode with the article on the support spaced from the electrode, means including a high-voltage source for maintaining an electrostatic field between said electrode and the article moving past it, means for introducing finely divided coating material into said field, and means including a member of conducting material carried by said support for reducing the strength of the field over a portion of. the surface to be coated.

1 WILLIAM A. STARKEY.

REFERENCES CITED UNITED STATES PATENTS Name Date Meston Oct. 26, 1937 Number 2,097,233

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