US2894175A - Apparatus for spray painting - Google Patents

Description

July 7, 1959 1.. J. LAM M APPARATUS FOR SPRAY PAINTING Original Filed March 25, 1949 few/1r J 6 9M Gttorneg United States Patent APPARATUS FOR SPRAY PAINTING Lewis J. Lamm, Harrison Township, Macomb County,

Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware original application March 25, 1949, Serial No. 83,505. grilled and this application April 19, 1954, Serial No.

4 Claims. (Cl. 317-3) This invention relates to electrostatic spray coating and, more particularly, to a method of and apparatus for 'charging particles of coating material for use in electrothe particles charging system and which does not employ the article to be coated as an electrode of the particle charging system.

The above and other objects of the invention will appear more fully in the following description and drawings wherein:

Fig. l is a sectional view of a particle charging electrode system in accordance with the invention, the section being taken along the line 1-1 of Fig. 2;

Fig. 2 is an end view showing the particle charging electrodes taken in the direction along the line 2-2 of Fig. l; and

Fig. 3 is a modification of the electrode system of Figs. 1 and 2.

Referring to Figs. 1 and 2, 60 is a ring of conducting material carrying pointed discharge electrodes 62 arranged in a circular configuration all pointing inwardly. The electrodes 62 are elongated electrical conductors which are pointed or have but a very short radius of curvature and thus promote electrical discharge when they are charged to a high potential. A second electrode 64 of conducting material and venturi shape is provided and maintained at ground potential to produce a current collecting electrode having negligible corona discharge under operating conditions. The members 60 and 64 are mechanically connected and electrically separated by means of insulator 66. A spray gun is shown at 68 which may be of either the type using a gaseous fluid as propellant or of the type using high pressure to produce atomization and propelling forces. This modification permits air to enter the space between 64 and 68 and thus reduce gas turbulence within the charging Zone.

A modification of the electrical discharge system is shown in Fig. 3. In this figure the current collecting electrode 70 is mechanically attached to but electrically separated from the electrode carrying ring 74 by means of insulator 72. The ring 74 carries a series of inwardly projecting pointed electrodes 76. The insulator 72, ring 74 and electrodes 76 are the same as the insulator 66, the ring 60 and the electrodes 62 illustrated above in connection with Figs. 1 and 2. In Fig. 3, the current carrying electrode 70 is mechanically and electrically connected to the spray gun 78. This spray gun may be of any conventional type similar to the above noted in connection with 63. The electrode system shown in Fig. 3 is mechanically attached to the end of the gun so as to form a unit assembly with the gun itself.

Operation (Figures 1, 2 and 3 When a conductor having a very small radius of curvature is subjected to a high electrical potential the concentration of electrical charge becomes sufiiciently great to break down the surface barrier and produce what may be called electrical discharge. This electrical discharge causes the ionization of the atmosphere immediately surrounding this small radius of curvature. The magnitude of the electrical discharge is a direct function of the potential applied to the electrode and an inverse function of the radius of curvature of said electrode. In the present case I use pointed electrodes to obtain the smallest possible radius of curvature and arrange these electrodes in a circle to concentrate the electrical discharge in the desired zone.

There must be a collecting electrode of some type to permit the space current to accumulate, and in the present invention I employ an auxiliary electrode which is separate and distinct from the articles to be coated and is fixedly spaced a slight distance from the discharge electrodes. It is very necessary that this collecting electrode have a very large radius of curvature if it is desired that no electrical discharge be produced at that point. In the present instance I desire no electrical discharge at the current collecting electrode. Therefore, I utilize current collecting electrodes of large radius of curvature and of substantial expanse forward of the gun and extending in the direction of particle travel to substantially elim inate all electrical discharge at these electrodes. Any electrical discharge at the current collecting electrode would be opposite in polarity to that of the pointed discharge electrode and therefore would tend to defeat the purpose for which the present system is designed. If minute particles of material are projected through this zone of electrical discharge, the ionized atmosphere will act as a good conductor of electricity and permit the migration of charges of electricity from the discharge electrode to the particles to form an electrical surface charge thereon. The coating material is projected by a spray gun either 68 or 78 through the electrical discharge zone formed at the points of the electrodes 62 or 76 and while in this zone are given an electrostatic charge of the same polarity as that to which the pointed electrodes are charged. The paint particles then proceed in the form of a charged cloud or spray to the coating area and are deposited on the articles of manufacture which are maintained at ground potential.

In apparatus such as the present type it is quite desirable that the zone of electrical discharge should not extend to the surface of the current collecting electrode.

If this Zone of electrical discharge with resultant ionization of the atmosphere extends to the grounded surface electrode, a good conducting medium is provided between the discharge and the collecting electrodes. This results in arcing perhaps with resulting fire if the material being sprayed is inflammable. For this reason it may be desirable to maintain the electrode system in an atmosphere which would not promote combustion. Such an atmosphere may be formed by the use of inert gas to replace a substantial percentage of the air normally used.

As shown in Fig. 1, the coating material projecting means has an elongated spray passage 68a therein, and it will be noted that the pointed ends of the normally directed discharge electrodes 62 are located outwardly from or beyond the peripheral edge of the projection of the elongated spray passage as shown by the dashed lines on the drawing, whereby the discharge electrodes will not be contained in or extend into the projected particle stream and become matted or coated with adhering particles that would impair the charging efficiency and necessi- 3 tate frequent system shutdown to permit cleaning of the charging electrodes.

By reason of the slight spacing or separation between the discharge and collector electrodes, considerably lower charging potentials are required with the particle charging system of the present invention than in those electrostatic depositing systems which employ the article to be coated as the space discharge current collector electrode. And since the discharge and collector electrodes are fixed in relation to each other, the electrostatic charging field appearing therebetween, when the electrodes are maintained at a difference of potential, will be of constant and uniform intensity and the particles of coating material will thus be charged uniformly. Lastly, since the projected particle stream passing through the charging system is surrounded by the ionized discharged from the circular array of discharge electrodes of the ring charger herein, substantially all of the projected particles will be charged thus assuring maximum charging efliciency.

I claim:

1. Apparatus for charging particles of coating material, comprising: a spray gun having an atomizing portion including an elongated spray tube with a bore for the passage of air and liquid; 21 current collecting electrode formed of electrically conductive material and located adjacent and forwardly of said spray tube and comprising a venturi shaped annulus having a bore of large surface area relative to the bore of said spray tube, the bore of said annulus being concentric with said spray tube and flaring outwardly as it extends forwardly; a discharge electrode comprising a ring of electrically conductive material located forwardly of said collecting electrode and an array of attenuated discharge members extending radially inwardly from said ring into and positioned generally normal to the path of travel of atomized particles issuing from said spray tube; and a ring of insulating material between said current collecting electrode and said discharge electrode.

2. Apparatus of the character claimed in claim 1, wherein the thickness of said ring of insulating material is greater than half the length of the bore of said annulus.

3. Apparatus of the character claimed in claim 2, wherein said spray tube is spaced from said current collecting electrode a distance less than the length of the bore in said current collecting electrode.

4. Apparatus of the character claimed in claim 2, wherein said spray tube abuts the end of said current collecting electrode and is mechanically secured thereto.

References Cited in the file of this patent UNITED STATES PATENTS 1,346,811 Diebold July 20, 1920 1,980,821 Paluefl? Nov. 13, 1934 2,302,185 Campbell Nov. 17, 1942 2,525,347 Gilman Oct. 10, 1950

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