US3463121A

US3463121A - Spray gun

Info

Publication number: US3463121A
Application number: US580468A
Authority: US
Inventors: Arvid C Walberg
Original assignee: HG Fischer and Co Inc
Current assignee: HG Fischer and Co Inc
Priority date: 1966-09-19
Filing date: 1966-09-19
Publication date: 1969-08-26
Anticipated expiration: 1986-08-26
Also published as: US3637420A; BE703225A; US3621815A; NO123926B; NL6712266A; FR1542729A; GB1173056A; DE1652324A1

Description

Aug. 26, 1969 A. c. WALBERG SPRAY GUN 2 Sheets-Sheet 1 Filed Sept. 19. 1966 INVENTOR AQVID C. WALBEQG mum, WHLG Aug. 26, 1969 A. c. WALBE'RG SPRAY GUN 2 Sheets-Sheet Filed Sept. 19, 1966 W mt g United States Patent 3,463,121 SPRAY GUN Arvid C. Walberg, Lombard, Ill., assignor to H. G. Fischer & Co., a corporation of Illinois Filed Sept. 19, 1966, Ser. No. 580,468 Int. Cl. Bb 5/02; F23d 11/28 U.S. Cl. 118-631 15 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to spraying systems for applying coating material and more particularly to systems which are capable of electrostatically depositing coating materials having water as a carrier.

Attempts to spray porcelain enamel electrostatically using water as a carrier have met with failure during the past fifteen years. The quality of finish has been unsatisfactory. The various electrostatic systems utilized causes defects such as (1) dry spray, (2) insufficient film thickness, (3) poor atomization, (4) non-uniform film thickness, (5) incomplete coverage and (6) poor electrostatic wrap-around.

Dry spray was caused by building the film thickness too slowly or by having the atomized spray travel exccssively long distances before it is deposited on the surface of the ware. The basic design of the systems used prevented the correction of the above problem. Since the liquid used in the porcelain enamel solution is water, the wetness or dryness cannot be adjusted in the same manner that solvent adjustments are made in electrostatic painting systems. When dry spray is encountered in painting, a slower wetter solvent can be added to the paint mixture. A- paint that is too wet can be corrected by adding a faster solvent. This adjustable feature is missing when water solutions are sprayed electrostatically and this factor prevented prior art electrostatic systems from properly applying water base materials.

Insufficient film thickness was applied in previous attempts to master the spraying of porcelain enamel electrostatically. A heavy paint film falls in the 1.0 to 1.5 mils range of film thickness. Equipment that was designed to apply films of this thickness was not adequate to meet the 3.0 to 6.0 mils of thickness specified for porcelain enamel. Poor atomization of the porcelain enamel occurred when attempting to atomize the larger volumes of material needed to apply the heavy film thickness indicated above.

Non-uniform film thickness was caused by stacking several electrostatic spray guns in a vertical plane to spray ware that was wider than the 12 inch maximum that could be handled by one gun. It was difiicult if not impossible to make the adjacent spray patterns together to avoid an improper boundary effect. If the adjacent spray patterns overlapped, there was a heavy build-up of enamel in the overlapped area. If the spray paterns fell short of meeting, a light streak could occur. The resulting quality was not acceptable.

Some attempts were made to overcome the above problem by oscillating the spray guns in a vertical plane. This moved the heavy and light streaks around somewhat, but it did not correct the basic problem.

3,463,121 Patented Aug. 26, 1969 1 CCv Incomplete coverage encountered with previous electrostatic systems was due to the excessive amount of waste caused by the poor operating efiiciency of these systems. Some systems had better charging characteristics and wasted less porcelain enamel but could not penetrate into recessed areas, corners, or other hard-to-reach areas. The net result was incomplete coverage and excessive amounts of manual touch-up to complete the job. The actual savings received, if any, was slight.

Poor electrostatic wrap-around caused by previous attempts in the spraying of porcelain enamel electrostatically resulted in poor coverage of the ware. The air spray guns used required excessively high atomizing air pressures to atomize the high volumes of porcelain enamel that had to be sprayed. This caused excessive spray velocity and reduced electrostatic wrap-around. Disetype atomizers, on the other hand, can not atomize electrically conductive materials by electrostatic forces alone and had to be operated at exceptionally high rotational speed to atomize the porcelain enamel centrifugally. This again resulted in high velocity that caused poor electrostatic wrap-around. For the above reasons, electrostatic systems that have proved successful in applying paints and similar coatings have proved unsuccessful when used to apply porcelain enamel.

One system used consisted of a standard air-atomizing spray gun spraying through an ionizing electrode charged to 100,000 volts at the grounded ware passing on a conveyor at right angles to the direction of the spray. The ionizing electrode consisted of two vertical tubes mounted 12" from the surface being sprayed and on 20" centers. A row of needle point ionizers were attached on 2" centers to the tubes on the side facing the ware. The needles pointed toward the ware. A spray gun or a station of several spray guns mounted in a vertical plane were arranged to spray between the two vertical electrodes. The guns were at ground potential and were positioned 12" behind the ionizing electrodes or about 24" from the grounded surface being sprayed. This 24" distance of travel of the atomized spray particles caused excessive drying of the spray particles resulting in a dry coat being applied to the product. There was insufiicient flow-out. The quality of finish was not acceptable. In addition, all of the aforementioned faults were encountered in this system.

A variation of the above system called for mounting the guns to spray substantially parallel to the ware moving along on the conveyor. The spray passed between an ionizing electrode and grounded ware and was deposited on the ware by the electrostatic field. The distance traveled by the spray was again excessive and the resulting finish was unsatisfactory due to dry spray. All of the other faults indicated above were also encountered.

Both of the above systems were also unsuccessful because of low operating efficiency. Too much enamel was wasted.

A newer electrostatic system consisted of a horizontal spinning disc reciprocating in a vertical plane with a conveyor carrying the ware in a loop around the disc. The disc was charged to 100,000 volts DC. and the porcelain enamel was distributed over the disc by centrifugal force to be atomized at the edge of the disc. The disc normally atomizes paint electrostatically but it will not atomize electrically conductive material electrostatically. Since porcelain enamel is electrically conductive, it was necessary to drive the disc at high speed to atomize the porcelain enamel by centrifugal force. This system was more efiicient since it atomized the material at the zone of ionization but the material was applied slowly as the ware passed around a 20 foot conveyor loop. This caused dry spray andrequired complete respraying of the surface with conventional air-atomizing spray guns to obtain an acceptable quality.

The disc type atomizer also has severe limitations on the volume of porcelain enamel that could be atomized per unit of time. This greatly limited material delivery and caused insuflicient film thickness and required excessive manual touch-up to complete the job.

A disc type atomizer had an additional disadvantage since it sprayed at all empty spaces between the products on the conveyor and caused excessive waste and uncontrolled buildup of coating on edges of a product that happened to be adjacent an open space.

The present invention overcomes a major share of the problems encountered in previous systems. Dry spray is not a problem. It was overcome by placing specially designed electrostatic spray guns at a much closer distance to the surface being sprayed. A spacing of 8" to was found satisfactory. This spacing would be unsafe if a solvent-based paint was used in the system. The entire film of several mils is applied in a relatively short distance of conveyor travel. The actual coating operation is complete in six feet of conveyor travel or less. The velocity range of conventional conveyors is from ten feet per minute to twenty-four feet per minute. Thus, the present invention applies the required film thickness in less than sixty seconds. This assures the desired wetness and the desired quality of finish.

Therefore it is a principal object to provide a new and improved electrostatic spraying system for spraying abrasive material.

Another object of the present invention is to provide an electrostatic spraying system which is capable of effectively spraying porcelain enamel or other highly abrasive material that requires control of the temperature of the atomizing air so as to counteract effect of humidity in the atmosphere and thereby control wetness or dryness of the applied film.

Further objects and advantages will become apparent from the following detailed description taken in connection with the accompanying drawings, in which:

FIGURE 1 is a perspective view of an automatic electrostatic spraying system embodying the present invention;

FIGURE 2 is a partial sectional drawing of the electrostatic spray gun illustrated in FIGURE 1; and

FIGURE 3 is a partial broken-away elevational drawing of the electrostatic spray gun illustrated in FIGURE 2 taken along the 44.

While this invention is susceptible of embodiment in many different forms, there will be described herein in detail an embodiment of the invention with the understanding that the present disclosures are to be considered as exemplifications of the principles of the invention and are not intended to limit the invention to the embodiment illustrated. The scope of the invention will be pointed out in the appended claims.

Referring now specifically to FIGURE 1, a conveyor 40 of any conventional design well known to those skilled in the art propels work to be coated, such as

panels

41 and 42, when they are connected to the conveyor 40 by means of

conductive hangers

43 and 44, respectively. The conveyor 40 is electrically connected to an electrical ground and therefore the work carried by the conveyor, such as

panels

41 and 42, are also connected to ground through the respective electrically conducting hangers, such as 43 and 44, and the conveyor 40. A hydraulic reciprocator generaly indicated at 50 has a base 51 upon which is mounted a hydraulic unit 52 and

vertical guide rails

53 and 54. A pair of pulleys 55 and 56 are rotatably mounted to the guide rails 53 and 54 near the top and bottom of the rails. The pulleys 55 and 56 have grooves shaped to receive a drive chain 57 which has its ends connected to a spray gun carriage 58. The carriage 58 has ball bearings mounted internally (not shown) which contact the

respective rails

53 and 54 in order that the earriage 58 may be smoothly guided in vertical movement by the guide rails. An analog control generally indicated at 60 is operatively connected to the chain 57 and has a pair of cams 61 and 62 for engaging a rabbit ear hydraulic four-way valve 63. The analog control 60 and the the four-way valve 63 reverse the direction of travel of the spray gun carriage at the top and bottom of its stroke. By moving the cams'closer together the unit will take a short stroke. The shortest stroke will be in the neighborhood of 12 inches. By moving the cams on the analog further apart, the length of the stroke is increased up to the maximum possible for the size of the reciprocator. The maximum stroke is approximately three feet less than the height of the reciprocator 50. The four-way valve 63 is connected to the hydraulic unit 52 by hydraulic lines 64 and 65. Another pair of hydraulic lines 66 and 67. are connected between the hydraulic unit and a hydraulic drive motor 68 which is connected to the pulley 56 to drive it, and thereby drive the chain 55 and the spray gun carriage 58. Since the hydraulic unit 52 may be any conventional source of hydraulic pressure well known to those skilled in the art, and since the valve 63 and drive motor 68 are connected and operated as is well known to those skilled in the art, this structure wil not bedescribed in detail.

A hollow cross arm 70 in the form of a tube of elec trically insulating material is rigidly secured in the spray gun carriage 58. The movement of the cross arm 70 is therefore controlled by the analog control 60. The speed of reciprocation is adjusted by a manual control 69 which is mounted on and connected to the hydraulic unit 52. A set of four spray guns 71-74 are clamped to the cross arm 70.

A high voltage power supply capable of supplying voltages generally in the range of 60,000 to 150,000 volts has its positive output terminal connected to electrical ground and its negative output terminal connected to the spray guns 71-74 by high voltage

electrical lines

81, 82, 83 and 84. The high voltage line 83 passes through the hollow cross arm 70 in order to reach gun 74 and the gun 73 via line 84.

A porcelain enamel pressure tank is mounted on a three-legged insulator stand 91. The tank 90 has a cover 92 which makes an air-tight seal for the tank 90 when screw clamps 93 and 94 are utilized to secure it. A source of air pressure (not shown) is connected to an air line 95 of electrical insulating material (such as nylon) which has a manually or remotely operated air regulator 96 placed therein. The air line 95 is connected to the tank cover 92 to supply air at controlled pressure to the interior of the tank. A fluid hose 97 of electrical insulating material (such as nylon) extends through the cover 92 downward into the tank 90 and is connected to each of the guns 71-74 to supply coating material thereto. It will be noted that the hose 97, after it supplies coating material to the guns 71 and 72 through short connecting branches, passes through the hollow cross arm 70 to reach

guns

73 and 74.

An air heater 100, which contains electrical heating elements (not shown), has these elements electrically connected to a source of electrical potential (not shown) by electrical leads 101 and 102. The air heater has its air passages connected to a source of high pressure air (not shown) through an air line 103, a manual air pressure regulator 104, and an air line 105, connected to the gun 74 by an air line 106. A similar atomizing air heater (not shown) is provided for each of the other spray guns also. A solenoid air valve is also connected to a source of high pressure air (not shown) by an air line 111, and it is'connected to the trigger mechanism of gun 74, as will presently be described in greater detail, through the air hose 112. Similar controls (not shown) are provided for

spray guns

71, 72 and 73 as well.

Since each of the spray guns 71-74 is identical in construction, only gun 71 will be described in detail. Re-

ferring now to FIGURES 2 and 3, the spray gun 71 comprises a metallic rear housing 120 and a barrel portion of nylon insulating material, generally indicated at 121, which comprises a first barrel section 122 threaded to a second barrel section 123 and an air cap 124 secured to the second barrel section 123 by a threaded annular retainer 125. The barrel section 122 is secured to section 120 by an annular fitting 126 to which an annular retainer 127 is secured by threads, and an annular cylinder 128 which is threaded onto section 122 and has an annular edge which contacts an annular edge of the retainer 127. A stainless steel valve 130 closes a passage 141 through the barrel portion 121 and the rear housing 120 when it engages an annular tapering valve seat 131 in a fluid tip 142. The valve 130 is connected to a stainless steel square shaft 129 which is connected to a stainless steel operating shaft 132 which is in turn connected to a stainless steel shaft 133.'Shaft 133 is connected to an adjustable needle plunger 134 which has a spring 135 urging it towards the forward portion of the spray gun. A second spring 136 bears against a spring retainer 137 which is rigidly mounted on the shaft 132 to urge the shaft 132 and the valve 130 forwardly, closing the valve against the valve seat. Water carried porcelain enamel coating material from hose 97 enters an aperture 140 that connects with the passage 141 containing the

shafts

129, 132 and 133 in order that coating material may flow around the shafts to the fluidtip 142. Thus, the coating material is conducted directly around the shafts which operate the valve in the front of the gun to the valve 130 within the fluid tip 142. Preferably, all parts with which abrasive coating material contacts such as water based porcelain enamel are constructed of abrasive resistant material such as tungsten carbide and stainless Steel. The fluid tip 142 is secured between the air cap 124 and a fluid tip housing 143. Sealing rings 144, 145 and 146 prevent fluid from the passage 141 from leaking past the fluid tip housing 143. The fluid tip 142 and the fluid tip housing 143 are constructed of tungsten carbide and stainless steel, respectively, so that abrasive coating material such as porcelain enamel will have a minimum erosion effect upon these parts. The high voltage electric lead 81 is connected through an electrical lead bushing 146 and an electrical truss screw 147. The air hose 106 is connected to the threaded fitting 148 to supply air to the passages generally indicated at M9 and 150. The passage 150 supplies air to two air horns 160 and 161 which form part of the air cap 124, and the passage 149 supplies air to an annular air passage 152 in the fluid tip 142.

The forward edge of the fluid tip 142 and a needle 170 mounted on the valve 130 and extending through the orifice of the fluid tip 142 form an electrode.

The length of the overall barrel 121 is normally of the order of 18 inches which prevents any substantial electrostatic field from existing between the charged rear housing 120 and grounded product 41. The strongest electrostatic field exists between fluid tip 142 and needle 170 toward grounded product 41.

A pilot bushing 165 aids in completing the passage 141 across the

junction barrel section

122 and 123, and a cylinder adapter 162 aids in completing this passage across the junction of the rear housing 120 and the barrel section 122.

The guns 71-74 are similar in construction and design to the hand gun shown in my U.S. Patent 3,251,551, issued May 17, 1966 with the primary diiferences being that harder metals are used for such parts as the fluid tip 142 to resist abrasion, and that electrical connection is made directly to the fluid tip through an aperture in the side of the barrel rather than having an electrical lead run from one end of the insulated portion of the gun to the other end. While the gun illustrated in my U.S. Patent 3,251,551, issued May 17, 1966 can be utilized for coating material having a resistivity of ten megohms or greater per inch cube, the guns illustrated in FIGURES 1-3 can spray electrostatically conductive materials such as water based materials and such as Teflon. When a coating material such as Teflon is sprayed, it is not necessary to utilize the air heater 100 in the system.

However, in spraying porcelain enamel, the air heater 100 is an important element of the system. Since porcelain enamel coatings require the use of water as a vehicle in the slip being sprayed, the relative humidity in the ambient air in the spray booth or spray area creates special problems in control of the operation. These problems are especially noticeable during the summer months when the variations in relative humidity are the greatest. The heater 100 increases the atomizing air temperature as the humidity of the ambient air increases. This evaporates moisture in the sprayed film of coating on the work at an increased rate to permit control of the wetness or dryness of the applied film. The operating temperature of the heater may be as high as the order of 210 F. The heater 100 can be either controlled by a manually operated control or by a humidistat. If a humidistat is utilized, a completely automatic system is achieved since the relative humidity in the spray area or booth can vary substantially and repeatedly with a resulting deterioration of the quality of the applied porcelain enamel.

Porcelain enamel is very abrasive and creates problems in regulators used to control its pressure. Standard pressure regulators presently used in the spray painting art will not stand up under porcelain enamel service. Therefore, the system illustrated in FIGURES 1-3 does not have a regulator between the tank and the spray gun, but instead utilizes a pressurized tank to eliminate the requirement of a regulator. By utilizing a stationary pressure source, there is a variation in the static head at the various gun elevations as the reciprocator raises and lowers the guns as the work passes through their spray patterns as illustrated in FIGURE 1. This problem is overcome by the utilization of an orifice that creates sufficient back pressure at the desired delivery rate to cause a relatively high pressure to be maintained at the pressure tank containing the porcelain enamel. Thus, the static head becomes a small percentage of the total fluid pressure at the spray gun. The static pressure of porcelain enamel varies approximately 0.65 to 0.75 p.s.i. for every foot of variation in elevation. When reciprocating the gun vertically, the gun will deliver more material at the bottom of the stroke and less at the top of the stroke. The film thickness on the product will vary in the same proportion. When a spray gun is positioned 36 inches above the fioor, for example, a pressure at the gun of three p.s.i. can be established. If the pressure is controlled by a stationary pressure tank, this pressure will remain fixed. When the 'gun is raised 12 inches, the pressure will decrease approximately 0.75 p.s.i., leaving only 2.25 p.s.i. At 60 inches above the floor, the pressure will be reduced to 1.5 p.s.i. Thus, with only a two-foot stroke, there would be a pressure variation of 2 to 1 and the applied film thickness would vary in a ratio of approximately 2 to 1. By using a fluid orifice sufiiciently small to cause a high tank pressure, the variation in film thickness from top to bottom of the panel will be greatly decreased. At 10 p.s.i. fluid pressure variation of 2 feet in elevation will cause a variation in static head of approximately 1.5 p.s.i. This is only 15 percent of the total and is therefore more acceptable. At 15 p.s.i. tank pressure, the variation drops to 10 percent and is considered highly satisfactory. Therefore, a tank pressure of 15 p.s.i. or higher is utilized to maintain a substantially consistent film thickness.

Spray guns 71-74 have the ability to atomize much larger volumes of material than anything encountered in the past. This permits the applying of heavy film thicknesses at high operating speed (3.0 mils or greater in less than sixty seconds) with a minimum amount of equipment. The fast build-up of film thickness also helps to provide the necessary wet coat and eliminates the problem of dry spray. With the guns mounted horizontally and reciprocated vertically for the full height rather than being stacked vertically as in previously systems, each gun can apply a minimum of two light coats of porcelain enamel. Therefore, the four guns can apply eight light coats in rapid succession. Thus, the system illustrated in FIGURES 13 can provide higher quality than was possible prior to the present invention.

While the spraying of porcelain enamel and Teflon in a water-base carrier has been specifically referred to, those skilled in the art will recognize that other coatings composed of particles such as various glass materials can be handled in the same manner as porcelain enamel, and that other particle coatings such as a plastic may be handled without humidity control in the same manner as Teflon. While abrasive material such as porcelain enamel can only practically be handled by air atomizing spray guns as previously described herein, less abrasive materials such as Teflon and similar material can be sprayed as herein described both by air and hydrostatic spray guns. The scope of the appended claims in intended to include all such variations and modifications.

If it is desirable in a particular installation to have an electrode which will be highly erosion resistant beyond the structure illustrated in FIGURE 2, the fluid tip 142 may be made of tungsten carbide or other similar erosion resistant material, and the needle 170 may be laterally offset from the nozzle of the fluid tip sufficiently so that material being ejected from the nozzle of the fluid tip does not impinge upon the needle. Such lateral offsetting of the needle 170 is accomplished by securing the needle to the fluid tip 142 rather than to the valve 130. Such modifications are intended to be within the scope of the appended claims.

I claim:

1. An electrostic coating material spraying system for porcelain enamel frit comprising:

an automatic air atomizing spray gun having a nozzle,

mounting means supporting said gun and electrically insulating it from ground potentials,

a container receiving a supply of porcelain enamel frit,

another mounting means supporting said container and insulating it from said ground potentials,

a conduit connecting said container to said spray gun,

means for moving work to be coated relative to said spray gun at a predetermined rate,

means operatively connected to said container, conduit and spray gun to force coating material from said container through said conduit and said spray gun nozzle at a rate suflicient to place a film of said material having a thickness of at least 3.0 mils on work to be coated as it is moved relative to said spray gun at said predetermined rate, and

high voltage power supply means connected between said gun and work to be coated to provide an electrostatic field between work to be coated and said gun. 2. An electrostatic coating material spraying system in accordance with claim 1, wherein all metal parts in said sprayv gun which contact said fluid material are constructed of abrasive resistant material.

3. An electrostatic coating material spraying system in accordance with claim 1, wherein said mounting means reciprocates said spray gun vertically for at least the full height of work to be coated.

4. An electrostatic coating material spraying system for porcelain enamel frit comprising:

a multiplicity of automatic air atomizing spray guns having nozzles,

mounting means supporting said guns in a horizontal line and electrically insulating them from ground potentials, said mounting means reciprocating said spray guns vertically for at least the full height of work to be coated,

a container electrically insulated from ground potentials receiving a supply of porcelain enamel frit,

a conduit electrically insulated from ground potentials connecting said container to said spray guns,

means for moving work to be coated relative to said spray guns at a predetermined rate,

means operatively connected to said container, conduit and spray guns to force coating from said container through said conduit and spray guns at a rate sufficient to place a film of said material having a thickness of at least 3.0 mils on work to be coated as it is moved relative to said spray guns at said predetermined rate, and

high voltage power supply means connected between said gun and work to be coated to provide an electrostatic field between work to be coated and said gun.

5. An electrostatic coating material spraying system for conductive fluid material comprising:

an air atomizing spray gun,

fluid coating supply means connected to said gun,

a source of compressed gas,

temperature control means,

means for operatively connecting said temperature control means between said source of compressed gas and said gun to control the temperature of a flow of gas from said source at its ejection from said spray gun, wherein said temperature control means is a heating means for raising the temperature of said flow of gas before its ejection from said spray gun, and

a humidistat conected to said heating means to automatically vary the temperature of said flow of gas in response to variations in the humidity of air surrounding work being coated by said spraying system.

6. In combination with the electrostatic coating material spraying system as specified in claim 5 a mounting means connected to said spray gun for reciprocating said spray gun vertically for at least the full height of work to be coated, and

a pressure means operatively connected to said spray gun to force coating material through said gun under a pressure of at least 15 psi.

7. An electrostatic coating material spraying system in accordance with claim 6, wherein said fluid coating supply means is a container electrically insulated from ground potentials.

8. An electrostatic spray gun for spraying porcelain enamel frit comprising:

a body having a forward end provided with a pas sage therein having an intake for connection to a container receiving a supply of porcelain enamel frit and an outlet through which coating material is ejected into the atmosphere from the forward end of the body, and with a passage therein having an intake for connection to a source of compressed air and orifices through which air is ejected from the forward end of the body to impinge upon coating material ejected from said outlet,

a circularly shaped nozzle of abrasive resistant material in said outlet, said nozzle and said air orifices providing means for air atomizing coating material on ejection from said outlet,

means applying an electrostatic field to coating material passing from the nozzle including an electrical conducting element adjacent said nozzle and extending forwardly of said nozzle, said element being sufficiently laterally offset from said nozzle to avoid material ejected from said outlet, and

means for charging said electrical conducting element to a high potential with respect to work to be coated.

9. An electrostatic coating material spraying system for porcelain enamel frit comprising:

an automatic air atomizing spray gun having a nozzle,

mounting means supporting said gun at a distance less than twelve inches from work to be coated Whenever the gun is operated to coat the work and electrically insulating said gun from ground potentials,

a container receiving a supply of porcelain enamel frit,

another mounting means supporting said container and insualting it from ground potentials,

a conduit connecting said container to said spray gun,

means for moving the work to be coated relative to said spray gun at a predetermined rate,

means operatively connected to said container, conduit and spray gun to force coating material from said container through said conduit and said spray gun nozzle at a rate sufiicient to place a film of said material having a thickness of at least 3.0 mils on the work to be coated as the work is moved relative to said spray gun at a predetermined rate, and

high voltage power supply means connected between said gun and the work to be coated to provide an electrostatic field between the work and said gun.

It). An electrostatic coating material spraying system in accordance with claim 1, wherein said film has a thickness of at least 3.0 mils applied to said work to be coated in less than sixty seconds.

11. An electrostatic coating material spraying system in accordance with claim 9, wherein said film has a thickness of at least 3.0 mils applied to said work to be coated in less than sixty seconds.

12. An electrostatic coating material spraying system in accordance with claim 1, wherein said material cornprises porcelain enamel frit and water.

13. An electrostatic coating material spraying system in accordance with claim 4, wherein said pressure means maintains a fluid material pressure greater than 15 p.s.i. in said gun.

14. An electrostatic coating material spraying system in accordance with claim 4, wherein the variation in the static pressure of the fluid material in said gun does not exceed fifteen percent of the maximum pressure utilized during the reciprocation of said spray guns by said mounting means.

15. An electrostatic coating material spraying system in accordance with claim 9, wherein said gun ejects said fluid material into the atmosphere at a distance from said work to be coated of not greater than ten inches and not less than eight inches.

References Cited UNITED STATES PATENTS 2,546,701 7/1951 Ransburg et al. 118631 2,736,671 2/1956 Ransburg et a1. 118-631 XR 2,780,565 2/1957 Juvinall 118-626 XR 2,893,894 7/1959 Ransburg 118626 XR 3,169,882 2/1965 Juvinall et a1. 118-627 XR 3,169,883 2/1965 Juvinall 239-3 XR 3,248,059 4/1966 Fischer et al 239- 3,251,551 5/1966 Walberg 118626 XR FOREIGN PATENTS 964,339 7/ 1964 Great Britain.

PETER FELDMAN, Primary Examiner US. Cl. X.R. 239--15