US 2698814 A
Description (OCR text may contain errors)
Jan. 4, 1955 E. M. RANSBURG 2,698,814
ELECTROSTATIC COATING APPARATUS AND METHOD Filed May 1, 1952 INVENTOR.
EDWIN M. RANSBURG WWW? A rforneys United States Patent ELECTRQSTATIC COATING APPARATUS AND METHOD Edwin lv Ransburg, Indianapolis, Ind., assignor to Ransburg Electra-Coating Corp., a corporation of Indiana Application May 1, 1952, Serial No. 285,575
5 Claims. (Cl. 117-62) This invention relates to electrostatic coating and more particularly to coating apparatus and coating methods to be used in electrostatic coating systems.
In a majority of electrostatic coating systems a field of electrostatic force is set up which includes the article being coated and the atomized coating material particles in movement toward such article. Deposition of the atomized particles is primarily the result of electrostatic forces and the particles are given an electrostatic charge at the time they leave the atomizing or spraying device or shortly thereafter. Where the articles are of conducting material, it has been the normal commercial practice to mount the articles on a grounded conveyor and thus to make the articles themselves an electrode at ground potential. Another electrode at high electrical potential is provided, with this electrode sometimes being the spraying device and sometimes being a separate electrode near or past which the spray particles pass in their movement toward the article.
When electrostatic coating methods are used to apply coating material to articles made of non-conductive material, it is often extremely difiicult to cause such articles to become a collecting electrode in the electrostatic field so that the sprayed particles of coating material are precipitated on the article through electrostatic forces. Various techniques, such as backing the article with a conducting member or heating or wetting the article so as to cause them to become conductors, have been tried with varying degrees of success.
The principal object of the present invention is to provide a method and apparatus whereby non-conductive articles can readily be coated in an electrostatic coating system. To this end the invention contemplates exposing the surface of the article to be coated to an electrode carrying an electrostatic charge so as to provide such surface wth an electrostatic charge differential relative to the charged particles. Where deposition of coating material particles is by electrostatic forces from an electrode at or near the atomizing means, positioning of a non-conducting article in depositing relation to the field thereby created causes the buildup of an electrostatic charge of the same polarity as the electrode on the surface of the article and thus, before the desired amount of coating is deposited, causes the article to repel the charged particles. Thus only a very thin unsatisfactory coating could be applied to non-conductive surfaces with ordinary electrostatic coating systems. According to the present invention, however, means are provided for removing charges so built up on the article surface so as to maintain a charge differential between the surface and the charged particles and thus to permit the application of a coating of desired thickness. In the preferred form of invention illustrated and hereinafter to be described in detail, the articles are rotated and carried along a conveyor past an ionizing electrode which is positioned adjacent the path of movement of the articles so that electrostatic charges are deposited on the surface of the article confronting the electrode. Positioned on the opposite side of the article is an atomizing means for atomizing fine particles of the coating material together with means for creating a charge on the atomized particles having a polarity opposite to the polarity of the electrode. Asthe articles continue to rotate during the coating operation, that portion of the surface of the article confronted by the electrode is rotated away from the electrode and toward the atomizing means so as to bring the charged portion of the surface into field creating relationship with the charged particles and thus to cause their deposition thereon. Continued rotation of the article brings the coated portion again near the electrode to remove the charge deposited thereon during the deposition of coating material and thus conditions such portion to receive further coating as rotation of the article continues.
Other features and advantages of the invention will be readily apparent from the following description and drawings, in which:
Fig. l is a perspective view of an apparatus embodying the invention; and V Fig. 2 is a diagrammatic View serving to illustrate the manner in which the invention is utilized.
While one form of apparatus is illustrated in the accompanying drawings, and will be described hereafter as providing means for practicing the present invention, it will be understood that this is a representative embodi' ment only. It will also be understood that forms of spraying devices other than the particular electrostatic atomizing device shown may be utilized and that other apparatus may be employed for accomplishing the purposes and for practicing the method of the invention, and it is to be understood that other embodiments may be utilized without departing from the contemplated scope of the present invention and that no limitations arg :10 be implied from the specific description now provr e In the exemplary embodiment of the invention illustrated in Fig. 1, there is shown a conveyor 10 for carrying a plurality of articles 11 along a predetermined path. An ionizing electrode 12 is positioned on one side of the articles adjacent the path, and an atomizing means 13 is also positioned adjacent the path but on the opposite side of the articles. The articles 11 are shown as cylindrical tubes which may be of Bakelite, cardboard, paper, or other similar non-conductive material. The atticles are suspended by means of hooks 14 from a rotator 15 which is attached to a pendant 16 movably carried on rollers (not shown) traveling inside of a hollow beam member 17. The rotator 15 is provided with a cylindrical surface adapted to contact a rotator bar 18 carried on brackets 19 by the beam 17 and serves, upon movement of the pendant 16 along the conveyor,'to rotate the rotator and hence the article.
The electrode 12, as previously noted, is provided for the purpose of conditioning the confronting .nonconductive surface for the reception of charged coating material particles. Not only may such conditioning be achieved initially through the action of the electrode in creating on the confronting surface of the article a charge opposite to the charge on the particles, but also the electrode serves to remove electrostatic charges from the surface during the coating operation. For the latter purpose the electrode may take many forms including that of the ionizing electrode shown, or the electrode may be in the form of a charged rod or plate and if in such latter forms, should be spaced very closely to the article.
The ionizing electrode 12 shown comprises a plurality of discharge elements 20 secured to a vertical upstanding support bar 21, with the discharge elements being tapered to needle-like points at their outer ends. The
ionizing electrode 12 is supported on a fixed column 22 of insulating material so that it can be maintained at a high potential with respect to ground. A. power Pack 23 has one terminal 24 connected by means of the conductor 25 to the ionizing electrode, with the opposite terminal of the power pack being connected to ground at 26. The conveyor itself is grounded as indicated at 27. The discharge elements 2d of the ionizing elecnode 12 are spaced from the surface of the article 11 at a distance which will allow the articles to be carried on the conveyor past the outer pointed ends of the discharge elements without contacting such elements. Preferably, the elements 2% are positioned rather close to the articlewith the space, of course, being sut'rlcient to prevent a spark from taking place between the electrode and the article or its support.
Located on the opposite side of the conveyor from the ionizing electrode 12 is the atomizing means 13. While various types of atomizing means may be utilized in the system herein disclosed, for the purpose of exemplary embodiment the atomizing means is illustrated as being of the highly efiicient rotating bell type such as more fully shown and described in the copending application of Edwin M. Ransburg, filed February 13, 1950 as Serial No. 143,994. As is more fully described in said application, the atomizing means comprises a bell-shaped head 30 supported on a hollow shaft with the shaft in turn being rotatably supported in a housing 32 carried by an insulating column 33. A second power pack 34 has its hot terminal 35 connected by means of the lead 36 to the housing of the atomizing means, with the opposite terminal of the power pack being grounded as indicated at 37. Coating material is supplied to the interior of the hollow shaft through a plastic or other insulating type tubing 38. The hollow shaft opens to the interior face of the bell and rotation of the bell serves to spread the material in a thin film over the interior surface of the bell for atomization from the edge thereof.
As will be noted from the drawings, the ionizing electrode 12 is charged oppositely to the charge on the atomizing means. Thus, the ionizing electrode may be positively charged while the atomizing means 13 is negatively charged. It will also be noted that the atomizing means is spaced considerably farther from the article than is the ionizing electrode. Such an arrangement permits proper dispersion of the coating material particles as they move toward the article.
Referring now to Fig. 2, the electrically charged ionizing electrode 12 causes the deposition on the confronting surface of the article 11 of a plurality of charges, diagrammatically indicated at 40, which accumulate thereon to create an electrostatic charge over the surface of the article. The charge is of predetermined polarity, in the case illustrated positive, and continued rotation of the article brings the portion of the surface so charged into confronting relationship with the atomizing means. The particles from the atomizing means are charged oppositely to the charge on the surface of the article, in this case negatively, and thus are attracted to the surface by the electrostatic forces created between them. Passage of the surface of the article in field creating relationship to the atomizing means tends to create on the surface an electrostatic charge of the same polarity as the atomizing means and thus, in the example illustrated, tends to create on the surface a negative charge such as illustrated at 41. A negatively charged surface would, of course, tend to repel the negatively charged particles if re-exposed to a spray thereof but in the system shown the negative charges are at least partially removed from the surface as the surface again rotates past the electrode 12. It is not essential that all negative charges be removed from the surface in order to effect subsequent deposition, but only that a charge differential exist between the surface and the particles sufficient to cause the deposition of the particles thereon by the electrostatic forces. It is not essential that electrode 12 be maintained at an electrical potential at the opposite side of ground from the spray particles in order that the apparatus be operative, as the electrode 12 may be at ground potential or even slightly charged on the same side of ground as the spray so long as its potential relative to the surface of article 11 is such as to create on said surface an electrostatic charge differential relative to the spray particles sufiicient to effect deposition of the particles. As another alternative, head 13 may be maintained at ground potential in which event the electrode 12 should be at a high potential, either positive or negative, in order to apply a charge to the surface of article 11 so that an electrostatic field will be established between said surface and head 13 which will be effective to obtain electrostatic deposition of the spray particles as the surface is moved into coating position.
It can be seen that the ionizing electrode may serve not only initially to create a charge on the surface of the article, but also, where the article is rotated several times during deposition, serves to remove negative charges from the surface which have accumulated thereon during the deposition of the charged particles, thus to leave the surface, as it is again rotated into depositing relationship, with an electrostatic charge differential relative to the charged particles.
A single electrode and atomizing means may be sufiicient to completely and uniformly coat the surface of an article made of non-conductive material. If the article, while positioned generally between the electrode 12 and atomizing means 13, is moved slowly along the conveyor or such movement is stopped, multiple coats may be applied by rotating the article through multiple revolutions while in such intermediate position. If the article is moved more rapidly along the conveyor, it may be necessary to provide additional electrodes 12 spaced along the path of article movement in order that the proper charge differential be created on the surface each time the article rotates in order that multiple coats may be applied, and in such cases it may also be found desirable to provide additional atomizing means along the path.
It will be clear that each time the article rotates to position a portion of its surface in confronting relationship to an electrode 12 a charge differential relative to the charged particles will be created on the surface of the article, which subsequently serves to attract the particles to the surface as the surface is again rotated into depositing relationship with the charged particles. Thus multiple revolutions produce multiple coats. By so utilizing the apparatus, multiple thin coats may be applied to produce an ultimate coating of the desired thickness. Such a multiple layer coating may, with some coating materials, produce a better over-all coating.
While there has been shown and described an exemplary embodiment of the invention, no unnecessary limitations of the scope of the invention are intended from such detailed description, such scope being indicated in the following claims.
1. The method of coating a non-conductive surface of an article in an electrostatic coating system which comprises exposing the surface to a spray of coating material particles carrying an electrostatic charge to deposit on said surface at least a portion of said particles, moving the surface carrying the deposited particles out of said spray and into confronting relationship with an ionizing discharge electrode carrying an electrostatic charge opposite to the charged particles to remove from said surface at least a portion of the electrostatic charge applied thereon during the deposition of coating material particles and to create on said surface an electrostatic charge differential relative to the charge on the particles of sufiicient strength as to be capable of electrostatically attracting said particles to and depositing them on said surface, and then moving said surface away from said electrode and back into said spray of coating material particles again to deposit thereon particles of the coating material.
2. The method of coating a non-conductive surface of an article in an electrostatic coating system which comprises rotating the article to be coated, depositing on the surface of the rotating article electrostatic charges of predetermined polarity from a stationary ionizing discharge electrode confronting a portion of the non-eonductive surface of the article whereby to accumulate on said confronting surface an electrostatic charge, continuing to rotate the article to expose the surface so charged to a spray of coating material particles carrying a charge relatively opposite to the charged surface, the charge differential between the particles and the surface being of suflicient strength as to be capable of electrostatically attracting said particles to and depositing them on said surface, then continuing said rotation to return said surface into confronting relationship with said elec trode again to provide on said surface an electrostatic charge differential relative to the particle charge, and then continuing said rotation to again expose the charged surface to said spray of coating material.
3. The method of coating a non-conductive surface of an article in an electrostatic coating system which comprises moving the article into a position intermediate a stationary eletcrode maintained at high electric potential and a spray of coating material particles carrying an electrostatic charge relatively opposite to the electrode and rotating the article while in such intermediate position to expose said surface to said spray a plurality of times and to bring said surface into confronting relationship With the electrode intermediate the exposures to remove therefrom at least a portion of the electrostatic charge deposited thereon by the particles and to create thereon an electrostatic charge differential relative to the charge of the particles of sufficient strength as to be capable of electrostatically attracting said particles to and depositing them on said surface.
4. Apparatus for electrostatically coating a non-conductive surface of an article comprising means for moving the article along a predetermined path, an ionizing discharge electrode positioned adjacent to and spaced from said path, means for maintaining the electrode at high electric potential of predetermined polarity, electrostatic atomizing means adjacent said path for atomizing coating material and including another electrode from creating an electrostatic charge on the atomized particles of opposite polarity to the ionizing discharge electrode, and means for rotating the article repeatedly to bring said surface alternately into confronting relationship with the ionizing discharge electrode to create between said surface and the atomized particles an electrostatic charge differential of sufl'icient strength as to be capable of electrostatically attracting said particles to and depositing them on said surface and then with the atomizing means.
5. Apparatus for electrostatically coating a non-conductive surface of an article comprising a conveyor for moving the article along a predetermined path, means on the conveyor for rotating the article as it is moved along said path, an ionizing discharge electrode located in fixed position closely adjacent to and on one side of said path, means for maintaining the electrode at high electric potential, atomizing means adjacent to said path and positioned oppositely to the electrode, and another electrode spaced from said path a greater distance than the first mentioned electrode for creating an electrostatic charge on the atomized particles of opposite polarity to the ionizing discharge electrode to create between the atomized particles and said surface an electrostatic charge differential effective to deposit the particles on said surface, said rotating means being adapted to rotate the article a plurality of times as it passes between said electrodes repeatedly to bring said surface alternately into confronting relationship with said ionizing discharge electrode and then with the other electrode.
References Cited in the file of this patent 'UNTIED STATES PATENTS 2,187,306 Formhals Jan. 16, 1940 2,221,338 Wintermute Nov. 12, 1940 2,287,837 Smyser June 30, 1942 2,334,648 Ransburg Nov. 16, 1943 2,562,358 Huebner July 31, 1951 2,604,870 Blood July 29, 1952