US2989241A - Apparatus for electrostatic spray coating - Google Patents

Description

2 Sheets-Sheet 1 INVENTOR.

June 20, 1961 D. H. BADGER APPARATUS FOR ELECTROSTATIC SPRAY COATING Filed May 22, 1957 DAV/D H. 951065,? BY/vwyg 2m Altar e s l i June zo, 1961 D. H. BADGER 2,989,241v

APPARATUS FOR ELECTROSTATIC SPRAY COATING Filed May 22, 1957 2 Sheets-Sheet 2 Alto ne s Unite States Patent O 2,989,241 APPARATUS FOR ELECTROSTATIC SPRAY COATING David H. Badger, Indianapolis, Ind., assgnor to Ransburg Electro-Coating Corp., Indianapolis, Ind., a corporation of Indiana Filed May 22, 1957, Ser. No. 660,904

7 Claims. (Cl. 239-15) Y This invention is directed to apparatus forclectrostatic spray coating and particularly to improvements in apparatus employing electrically charged atomizers wherein liquid coating material is atomized from an annular edge.

Spray guns are known possessing an electrically charged annular atomizing edge which produce -aspray of finely divided charged liquid particles in an annular or doughnut-shaped pattern having a central portion substantially devoid of spray particles. Various means have been suggested for reducing the overall size and improving the yuniformity of such annular sprays and for minimizing the central void portion of these sprays. One suggested arrangement involved the use of asnout-like electrical probe consisting of a section of -wire slightly protruding from an insulating sheath located coaxially of and extending several inches forwardly of the annular atomizing edge of the gun. This probe was maintained at a voltage intermediate that of the highly charged atomizing edge and the articles which were commonly grounded.

Such probes, while effecting a distinct improvement over guns not using a probe, `possessed certain disadvantages. The probing action, that is, the -ability to confine the spray and reduce the size of the central void portion thereof was not too effective, especially at the closer edge-article spacings. Moreover, in a hand operable electrostatic gun designed to permit the gun to be safely brought close to or even in contact with the article being coated, the physical projection of a probe several inches beyond the atomizing edge tended to restrict free use of the gun.- In instances where the operator might desire to bring the gun within a few inches of thearticle to be coated, the probe interfered.

I have invented an improved electrical probe for confining and improving the uniformity of the charged spray from an annular atomizing edge and for reducing the size of the central void in such spray. My invention contemplates a probe somewhat smaller than the atomizing edge and located axially close to the plane of such edge, and

preferably in the form of a concavely dished disc located coaxially of the atomizing edge, entirely rearwardly thereof, and with the outer edge of the disc closely adjacent the atomizing edge. The concave disc may be of nylon or other highly insulating plastic material coated with a semi-conducting resistive coating composition. The disc is maintained at an appropriate potential intermediate the potential of the annular atomizing edge and the articles being coated.

My invention will be fully understood from the following detailed description and the attached drawings, in which:

FIG. 1 is a side elevational view, partially broken away, of an electrostatic hand spray gun utilizing one form of my invention;

FIG. 2 is a `detailed isometric view of the concave disc and rotatable atomizer shown in FIG. 1; and

FIG. 3 is a circuit diagram showing one electrical arrangement of my invention.

T o accomplish its function of increasing uniformity and decreasing the size of an annular spray, an electrical probe must distort the eld so that the electrical force acting on the charged spray particles has a substantial radial component directed toward the center of the pattern. A

2,989,241 Patented June 20, 1961` ICC conducting or semi-conducting surface placed near the atomizing edge and so shaped and charged as to increase the inwardly directed radial component of the electrostatic field intensity at the atomizingl edge will serve to oppose both the initial centrifugal force which is customarily imparted to the particles of liquid coating material as they leave the atomizing edge by rotation of the edge about its axis and reduce the overall divergence of lines of force extending from the edge to the article. The use of anelectrode concave in shape and having a relatively narrow edge portion coaxial with and closely adjacent the annular atomizing edge has proven effective for this purpose.

A limitation on the effectiveness of an electrical probe is the tendency of the probe to attract spray particles to itself and thus, become fouled with the liquid coating material intended for the article. Thus the shape, position yand electrical characteristics of the probe must be designed to reduce, and preferably eliminate, deposition of paint particles on the probe itself. Tests have indicated that a concave disc exhibits less tendency in this respect than do other similarly effective probes having a surface relatively close to the atomizing edge.

A further factor to be considered, particularly in reference to a hand operable gun, is the preferability that the entire apparatus including the probe be shock proof and spark proof. Electrostatic spray coating systems are known which are free from objectionable electrical discharges of such intensity that they could result in a disagreeable shock to an operator or a spark capable of causing a fire or explosion. Essentially the system is rendered shock proof and spark proof by limiting the amount of current available at the exposed portions of the spray gun and by reducing the effective capacity of the gun, and thus neither the flow of current from the voltage source nor the discharge of the capacity of thev electrical system can produce an undesirable effect. To meet ythe requirements of such a safe system it is necessary to. use a probe Whose ability to discharge its stored made of moulded epoxy resin #and a metal handle as-` sembly 12. A flexible high voltage line 14, a low voltage line 15 and a liquid feed line 16 are connected to the rear end of gun 10 and for convenience the three flexible lines are contained within a protective sheath 18.

High voltage line 14 consists of an insulation covered metal wire which is protected by a length of insulating tubing 19 extending through housing 11 and preferably somewhat rearwardly thereof. High voltage is supplied i to the hand gun from a voltage source 20 through line 14 whose forward end is connected to a high resistance 22 which may be in the order of 1000 megohms. The forward end of resistance 22 is connected to a small wire 24 making friction contact with a resistive coating 25a on the outer surface of a rotatable annular-edged atomizer 25. The resistive coating 25a is preferably a glyptal alkyd resin intimately mixed with a predetermined amount of finely divided carbon. This composition is thermosetting and when applied to an insulated base and baked the resulting coating exhibits high chemical and'I physical resistance to the ingredients of commonly used f liquid coating materials and also has relatively constant 1 electrical characteristics rendering any electrical energy stored by the coating impossible of instantaneous discharge.

Power for rotation of atomizer 25 is supplied by a small electric motor 26 within housing 11. Current is supplied to motor 26 through low voltage'line 15 andfthe motor is grounded through handle assembly 12; The motor is controlled by an on-off switch 2S and the shaft of the motor is drivingly connected to a rotatable shaft 30 on the forward end of which is mounted` a hub 31. Atomizer 25 is supported concentrically about hub 31 and spaced from it by a plurality of pins 33 in order to provide an annular opening 35 between the outer edge of the hub and the inner liquid-guiding surface 25bof the atomizer. Surface 25b terminates in a'forwardly projecting annular atomizing edge 25C. Atomizer 25, shaft 30, hub 31 and pins 33 are all constructed of nylon or the like. Operation of motor 26 causes atomizer 25 to rotate concentrically about the axis of shaft 30"pref erably at speeds in the order of 600 r.p.m.

Liquid coating material is supplied from a pressurized source to hand gun throughfeed line 16. A trigger 36 on handle assembly 12 controls the flow of liquid from line through a passageway in gun 10. A nozzle (not shown) projecting from the forward end of housing 11 discharges a stream of liquid onto the interior surface 25b of atomizer 25 so that the liquid spreads into a' thin lm owing through annular opening 35 toward atomizing edge 25C.

The electrical probe consists of a concave disc'38.of nylon or the like mounted on the forward surface of hub 31. Preferably disc 38 has a cylindrical edge portion 38a with relatively sharp edges located slightly rearward# ly of and closely adjacent to atomizing edge 25e. The entire disc 38, front and rear, is covered with a'coating '38h of semiconducting material, conveniently the same composition as that used in resistive coating 25a. The disc is electrically connected to a high resistance 40 in the order of 10,000 megohms by spring 42 and connector 43. The rear end of resistance 40 is connected to ground through motor 26 and handle assembly 12.

In operation, gun 10 is held in the hand and spaced from an article 45 which is maintained at ground `potential. resistances of values as previously specified, is supplied to the gun by voltage source 20. By energizing motor 26 the atomizer is set in rotation and by operation of trigger 36 liquid coating material is fed `through the gun to the rotating atomizer.

Due to the voltage drop across resistance 22 and coat? ing 25a the voltage at edge 25e will be lower than that of voltage source and when paint is supplied to edge e and the gun approximately 12 inches from grounded article 45, the voltage at edge 25e` will be approximately 70 kilovolts. Adjacent to edge 25C the liquid is charged substantially to the potential of the edge and the liquid atomized and projected into the electrostatic field as a charged spray. Although the liquid in feed line 16 may be at or near ground potential by contact with the grounded handle assembly 12, the majority of com# mercial liquid coating materials are relatively poor conductors so that the liquid column from edge 25C to handle assembly 12 does not provide a highly conductive path to ground.

The electrical attraction between the lcharged liquidground; and as the result of the voltage drop across the resistance the disc will be at a potential-intermediate that High voltage, in the order of 120 kilovolts with of the atomizing edge and ground. (The resistance of coating 38b' is so small in comparison with that of resistance 40 that its effect may for most purposes be ignored.) Accordingly, the presence of intermediately charged disc 38 provides a substantial inwardly directed radial component tothe electric field closely adjacent to the atomizing edge 25e. Spray particles are thus urged toward the center of the spray pattern substantially simultaneously with their atomization and projection from the liquid body. The probing action serves to reduce the overall size ofthespray pattern,` improve its uniformity and substantially reduce its central void portion. The spray particles are urged inwardly with insuicient force" to actually deposita substantial amount of spray particles onto disc 38 and the position of the disc entirely rearwardly of edge' 25e and itsiconcave shape contribute'to its freedom from deposited spray particles.`

An electrical probe made in accordance With'my"invention having a disc diameter of 2% inches and used in conjunction with a4 inch diameter atomizer and employing a resistance 40 of 10,000 megohms, produced significant improvements in pattern size and uniformity as compared withv the' snout-like forwardly projecting probes previously known. Asshown the probe edge 38a lies about 1/8 inch rearwardly of the plane of atomizer edge 25C. Any advancement of the probe forwardly tends to'increase paint deposition on the probe. However, with larger atomizers, producing increased centrifugal force, it may be desirable to bring the probe somewhat forwardly of the atomizing edge.

l claim:

l. In an apparatus for electrostatically spray coating a grounded article, said apparatus including an atomizing device provided with an annular atomizing edge, a source of high electrical potential, means for connecting said source to the atomizing edge to maintain the edge at high voltage andmeans for feeding liquid coating material to said edge for projection therefrom as a spray ofliquid coating material particles in a pattern having at least initially acentral portion substantially devoid of spray particles, means for reducing the void comprising: an electrical probe having an end located coaxially within and generally adjacent the plane of said annular atomizing edge, and means including a high resistance electrically connected at one end to the probe and at its other end to ground for maintaining said probe'at an electrical potential intermediate Vthe high voltage of said atomizing edge and ground, electrostatically to cause at least some of the spray particles initially surrounding said central void portion to be attracted inwardly into said void portion.

2. In an apparatus for electrostatically spray coatinga grounded article, said `apparatus including an atomizing device provided with an annular atomizing edge, a source of high electrical potential, means for connecting said source to the atomizing edge to maintain the edge at high voltage and means for feeding liquid coating material to said edge for projection therefrom as a spray of liquid coating material particles in a pattern having at least initially a central'portion substantially devoid of spray particles, means for-reducing the void comprising: an annular edged electrical probe positioned substantially coaxially within said annular atomizing edge and close to the plane of said atomizing edge, insulated means for mounting said probe on the atomizing device, a high resistance of pre` determined value located within said atomizing device, means `electrically connecting said probe to one end of said high resistance, and means electrically connecting the other end of said high resistance to ground thereby to maintain said probe at a potential intermediate said atomizing edge and ground.

3. In an apparatus for electrostatically spray coating a grounded-article, said apparatus including an atomizing device provided with an annular atomizing edge, a source of `high `electrical potential, means for connecting said source to the atomizing edge to maintain the edge at high voltage and means for feeding liquid coating material to said edge for projection therefrom as a spray of liquid coating material particles in a pattern having at least initially a central portion substantially devoid of spray particles, means for reducing the void comprising: a concave disc of insulating material located within and rearwardly of said atomizing edge, said disc having a semi-conductive surface, and means for maintaining the semi-conductive surface portion of said disc at a potential intermediate that of said atomizing edge and ground thereby to divert inwardly into said central void portion at least some of the particles initially surrounding said portion.

4. In an apparatus for electrostatically spray coating a grounded article, said apparatus including an atomizing device provided with an annular atomizing edge, a source of high electrical potential, means for connecting said source to the atomizing edge to maintain the edge at high voltage and means for feeding liquid coating material to said edge for projection therefrom as a spray of liquid coating material particles in a pattern having at least initially a central portion substantially devoid of spray particles, means for reducing the void comprising: an electrical probe positioned closely adjacent the plane of said annular atomizing edge and substantially coaxially within said atomizing edge, said probe Ibeing semi-conducting and means for maintaining said pro-be at a spray particle-attracting potential to cause at least some of the spray particles initially surrounding said void central portion to be attracted inwardly into said void portion.

5. In an apparatus for electrostatically spray coating a grounded article, said apparatus including an atomizing device provided with an annular atomizing edge, a source of high electrical potential, means for connecting said source to the atomizing edge to maintain the edge at high voltage and means for feeding liquid coating material to said edge for projection therefrom as a spray of liquid coating material particles in a pattern having at least initially a central portion substantially devoid of spray particles, means for reducing the void comprising: an annular-edged probe positioned coaXially and entirely rearwardly of said atomizing edge, a member of insulating material connecting the probe to the atomizing device, a high resistance, means electrically connecting said probe to one end of said high resistance, and means connecting the other end of said high resistance to ground thereby to maintain said probe at an electrical potential intermediate that of the atomizing edge and ground to divert inwardly into said central void portion at least some of the particles initially surrounding said portion.

6. In an apparatus for electrostatically spray coating an article, said apparatus including an atomizing device having a rotatable annular atomizing edge, a source of high electrical potential, means for connecting said source to said edge to maintain the edge at high voltage, means for feeding liquid coating material to said atomizing device and rotating said atomizing edge to deliver coating material to said edge for projection therefrom as a spray of liquid coating material particles in a pattern having at least initially a central portion substantially devoid of spray particles, means for reducing the void comprising: a concave disc of insulating material coated with a semi-conductive composition, means for mounting said disc on said atomizing device within said annular atomizing edge, means connecting the disc to the edge to rotate the disc with said annular edge, a high resistance, means electrically connecting the semi-conductive surface of said disc to one end of said high resistance and means connecting the other end of said high resistance to ground, thereby to maintain said disc at a potential intermediate the atomizing edge and ground electrically to cause at least some of the spray particles initially surrounding said void central portion to be attracted inwardly into said void portion.

7. In apparatus for electrostatically spray coating an article, an atomizing device having a body portion, a handle on said body portion for manual manipulation thereof, an annular atomizing edge rotatably mounted at one end of said body portion, means carried by said body portion for rotating said annular edge about its axis, means including a source of high voltage connected to said annular edge for maintaining said edge at a high potential, means for feeding liquid coating material to said edge for atomization therefrom as an electrically charged spray of liquid particles in an annular pattern initially having a central void portion, an electrical probe having a semi-conducting surface, means for mounting said probe on said atomizing device so as to maintain said probe positioned substantially coaxially of and with the end of said probe being closely adjacent to the plane of said annular atomizing edge, and electrical means for maintaining said probe at a spray particle-attracting potential intermediate that of said atomizing edge and ground to cause at least some of the spray particles initially surrounding said void central portion to be attracted inwardly into said void portion.

References Cited in the tile of this patent UNITED STATES PATENTS 2,600,129 Richards July l0, 1952 2,784,114 Miller Mar. 5, 1957 FOREIGN PATENTS 710,852 Great Britain .Tune 23, 1954

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