US3268171A - Electrostatic coating system - Google Patents

Description

Aug. 23, 1966 A. c. WALBERG 3,268,171

ELECTROSTATIC COATING SYSTEM Original Filed July 19. 1962 allttgj.

United States Patent O 3,268,171 ELECTROSTATIC COATING SYSTEM Arvid C. Walberg, Lombard, Ill., assignor to H. G. Fischer d; Company, a corporation of Illinois Original application `Iuly 19, 1962, Ser. No. 211,006. Divided and this application Mar. 25, 1966, Ser. No.

6 Claims.

This invention relates -to an electrostatic coating system and is a division of my copend-ing application Serial No. 211,006, filed July 19, 1962, now abandoned, which is a continuation-in-part of my prior application Serial No. 827,587, filed July 16, 1959, now United States Patent 3,056,557, issued on October 2, 1962.

In the patent identified above, there is disclosed and claimed a gun for use in an electrostatic coating system so constructed that there is minimum exposure to atmosphere in the neighborhood of the gun discharge region of metal at high potential. In accordance with the invention disclosed in said applicati-on, a sharp edge or sharp tip of metal charged to a high poten-tial and disposed in the gun outlet region serves to define an intense electrostatic field normally extending toward the work t-o be coated.

A gun as described above in a system embodying the present invention will have unusually safe operating characteristics and be free from any tendency toward dangerous arcing in the event of a ilashover. A flashover in a conventional system will generally result in an arc having suicient body or energy to start a re or to be dangerous. In a system embodying the present invention, any ashover or discharge will be of a very weak character and incapable of seriously injuring or damaging any person or thing.

A system embodying the present invention includes a gun of any desired type or construction, providing that all metal parts of the gun, except as hereinafter set forth, which will be at high potential with respect to the work to be coated will be insulated from atmosphere by solid insulation such as Bakelite, polyethylene or lother plastic having sufficient dielectric value to prevent any signicant electrical discharge from such highly charged metal to atmosphere or to any grounded object. Such a gun will have exposed to atmosphere at the region of gun paint discharge a small electrical discharge tip of metal. This electrical discharge tip of metal can be a line wire or needle-like piece of metal or, within limits, may be a sharp annular metal knife edge comprising the paint discharge nozzle of the gun. In a system embodying the present invention, any metal discharge nozzle for paint having an inside diameter larger than about .028 is not desirable if maximum field strength and safe operation is the objective, as is generally true of hand guns. In such case, the nozzle is not relied upon for discharging electricity to the atmosphere, but instead, a separate sharp tip or thin wire is used. A system embodying the present invention includes in addition to the gun, as pointed ou-t above, a dropping resistor whose value is determined by the normal operating potential of the system With the gun. By normal operating potential is meant the potential causing a current drain of about .l milliampere to flow. A substantial portion of this current drain is the current discharged into atmosphere by the sharp discharge tip of the gun. Some current will flow back through the paint line from the gun to the source of coating material. In accordance with the invention, the dropping resistor should have a value of about 25 megohms per 10,000 volts within the voltage range hereinafter specified.

In a system utilizing the invention, the spacing be- ICC tween the discharge electrode of the gun and the grounded work to be coated will range anywhere from about 3 inches to no more than about 18 inches. As a rule, the electric potentials for operating systems of this character will nange from about 60,000 volts to as much as 150,000 volts, with about 100,000 volts being the normal upper limit.

It is desirable that electrical resistance of the coating material for an electrostatic manual gun should be in the following range as measured under the following conditions. Such high resistance coating material will permit the handle of the hand spray gun to be grounded to permit maximum operator safety and also to prevent the high voltage from shorting back through the paint column to the grounded handle and yet provide a sufciently low paint resistance to permit the paint to readily become charged electrosta-tically.

Between electrodes, each having an area of three square inches and having a separation between electrodes of one-quarter of an inch, ythe direct current resistance of the coating material is preferably adjusted to lie between one and two megohms, :although up to ve megohms may be satisfactory. This works out to be a resistivity from a bit over ten megohms to about seventy megohms per inch cube.

Automatic guns do not require a grounded connection to the gun body or handle, since the gun will not be held in the hand. By mounting the automatic gun on an insulated support and supplying paint through a high dielectric hose (such as nylon tubing) the minimum level of paint resist-ance can be greatly reduced. In fact, if the paint supply :tank is also mounted on insulators, paint with a resistance close to zero can be handled in the automatic system. Therefore in automatic electrostatic painting systems, the desired paint resistance r-ange will be zero t-o five megohms as measured above or a resistivity of zero to seventy megohms per inch cube. The adjustment of the resistance may be accomplished in the following manner. A

If the coating material has too low a resist-ance for an electrostatic manual gun, it may be thinned by -such materials as toluol, Xylol, and other well known thinners which are highly insulating. If the coating material has too high an electrical resistance for either an automatic or a manual spray gun, its resistance may be reduced by such thinners as acetone (or other ketones), acetates, Cellosolves, and certain alcohols, of which butyl is an example. These thinners have relatively loW electrical resistance and reduce the electrical resistance of the coating material. Y

In general, the determination of which thinner to us may be easily made by consulting tables showing the electrical resistance of various liquids or by actually measuring the electrical resistance of the thinners or using a thinner and mixing it with the coating material and then measuring the electrical resistance.

FIGURE 1 is a view of a system embodying the present invention; land FIGURE 2 is an enlarged partial `sectional View of the end of the spray gun shown in the system illustrated by FIGURE l.,

The gun shown in the iigure may be of -the hand or automatic type. The ygun may also be of the type wherein coating material is discharged rin conjunction with an air blast for atomizing or may be of the high pressure hydraulic type wherein the coating material is discharged to atmosphere under pressure of the order of a thousand pounds per square inch or more. In either of the above types of gun, atomization is principally mechanical.

Inasmueh as guns of the above types are well known, no detailed structural description thereof is 'deemed to be necessary except tor the fact that the gun has a minimum of metal at high potential exposed to atmosphere in the general region Where coating material is discharged. One example of an air pressure gun which may be used is disclosed and claimed in my previously identified patent.

Referring now to the drawing, the g-un generally illus trated there h-as barrel carrying nozzle assembly generally indicated by 112. Barrel 10 and nozzle assembly 12 has t-he outside thereof of electrically insulating material. Whithin the interior of b-arrel 10` and nozzle assembly -12 is a suitable means tor discharging air and coating material -in the region generally indicated by 13. As may be more easily seen in FIGURE 2, the suitable means for discharging Zair and coating material include a coating material passage 40 having an outlet generally indicated at 41, a nozzle 42, which is formed by a conical shaped valve seat 43` adjacent the nozzle 42, a conical shaped valve 44 on which an electrical conducting element 4S is secured, a cylindrical air passage 46 which has an outlet generally indicated at 47. Electrical conducting element 45 extends through said nozzle and terminates in a tip 16. The Ioutlet 47 converges towards the outlet 41 and the nozzle 42 in order that air under pressure ejected by the outlet 4t7 will converge with coating material ejected through the nozzle 42 or outlet 41. Horns |14 and 15 laterally oiset from the .axis of the lgun barrel have air jets Ifor directing blasts of air toward region 13. As is well known, the coating material is discharged into region `13: and the discharge is controlled by a suitable valve within or adjacent nozzle assembly 12.

Nozzle assembly 12 of the gun has metal tip 16 which projects into region 1,3'. Metal tip 16 is normally charged to a high potential with reference to ground or work 17 which is located at a suitable distance from the gun. Charged metal tip 16 may be the tip of a needle Valve member or may consist of any other sharp edged tip. As an example, an annular metal nozzle edge through which the coating material is discharged may also-be used providing it is small enough. In the case of a high pressure hydraulic gun, charged metal tip 16 may consist of a wire embedded in the insulating nozzle assembly and connected to a source of high potential. In all cases, however, it is understood that except for charged met-al edge or tip 16 which projects into the air a short distance and which has minimum surface area exposed to the air, all other metal within nozzle cap 12 and barrel 10 will be adequately insulated from Aair to prevent any substantial electrical discharge trom occurring except at tip 16. The rear of the gun may include handle portion 20 containing trigger 21 for operating valves to turn the gun on or ott. The rear portion of the gun may be of insulation or metal and if metal is used, it is preferred that such metal be at ground potential.

In accordance with the present invention, dropping resistor 23 is provided. Thisdropping resistor is provided with suitable insulation around the outside thereof. One resistor terminal is connected to the metal within nozzle assembly -12 lby heavily insulated Wire 24 passing from the resistor into the nozzle assembly. By virtue of this arrangement, the length of wire between highly charged tip 1.6 of the .gun and the adjacent dropping resistor terminal is reduced to Ia minimum. Furthermore, the short length of IWire 24 makes it unnecessary to provide any ground-shielding. Thus the electrical capacitance of the part of the system including charged .tip 16 and dropping resistor 23 on the one hand and ground on the other hand is reduced to a negligible value. ln practice, such a connecting wire will be less than 4 inches in length. Dropping resistor 23 which is heavily insulated on the outside has its other terminal connected to cable 26. Cable 26 is conventional high potential exible cable used in this industry and is usually in the tor-m of a coaxial cable from about 20 feet to as much as 50 feet in length having G" or Mi" polyethylene insulation between a central stranded conductor and an outer braided conductor, the latter being grounded.

While connecting wire 24 Will generally be quite short, there may be occasions when it may be desirable to have it as much as one or two feet in length. In such case, the tact that this connecting wire is unshielded and is heavily insulated will keep its capacitance .to ground so low that the increase in length will not be objectionable. With reg-ard to cable 26, as hereinafter pointed out, the length is quite substantial and in practice a good part of this cable will lie on or be close to supports at ground potential. A-s an example, a iloor will have suicient conductivity so that a cable resting on the floor will rest on a ground potential surface. Because of this, the presence of the grounded outer conductor in cable 26 is not completely necessary in order to obtain the advantages of the present invention. Even without a coaxial cable, the relatively thin layer of insulation and .the practical result of having a large part of the cable resting on the floor will endow the cable with sufficient capacitance to ground so that the present invention does improve the performmance of such a system.

The mechanical means for attaching resistor 23- to the gun may assume a wide Variety of forms and as here il lustrated, is represented by clamp 28. The forward end of the dropping resistor may be retained in position by the heavy insulation tor wire `24 or, if dired, a separate clamp may be provided.

Gun 20 will be provided with lair and coating material inlets. The entire gun including barrel 10 and resistor body 23 may, if desired, be housed within one unitary mass of insulation. Cable (26 goes to high potential power supply 30. This high potential power supply will have one terminal grounded and the high potential terminal connected to the central conductor of the coaxial cable. Power supply A30 provides a high direct potential, the value being as previously indicated. The power supply includes suitable means for controlling the output potental and current, as IWell as safety means tor preventing the potential from rising above an arbitrary limit, say, 150,000 volts, to prevent damage to equipment. As is well understood, power supply 30 will \be energerized trom a conventional alternating current power line.

Insofar as coating material is concerned, gun 20 will have flexible hose 32 connected to an inlet 48 of passage 40 and going to a source of coating material. As a rule, hose 32 will be of electrically insulating material such as rubber, polyethylene or other plastic material, Hose 31 is connected to an inlet 49 of passage 46 for supplying air under pressure and is supported by clamp 31a.

The valve of resisto-r 23 will depend upon the desired normal operating conditions of the system. Apart from any particular value of resistor 23, its location in cornbination with an, electrically insulated gun having minimum exposed metal at high potential imparts improved operating characteristics to an electrostatic coating system. Y It should be noted that the present invention does not limit the gun design to minimum amounts of metal. The only requirement is minimum exposure of highly charged metal to atmosphere in the region of gun discharge.

In the practical design of a spray gun, it is desirable to use met-al. This is true whether the gun relies upon air or hydraulic pressure for effecting atomization. It is Well known that coating materials frequently contain abrasives and the high Velocity of the coating material makes it necessary to use metal for the discharge nozzle of the gun to reduce wear. Also it may be desirable to change the nozzle size for various kinds of coatings and interchangeable nozzles should be threaded and made of metal to permit frequent and easy replacement. It is necessary to provide valve means, for controlling the flow of coating material and air. Valve parts made of metal are more serviceable. In the case of hydraulic guns operating under high pressure, metal members are necessary except for the discharge nozzle where very hard materials such as tungsten Carbine are utilized. In all cases, it is easier to use metal than insulating materials.

Generally in various types of guns, it is `desirable to have the valve for controlling the flow of coating material as close to the discharge nozzle as possible. This avoids the -dribbling of coating material from the gun .after a valve is closed to shut the gun off.

The value of dropping resistor is too high to permit current to be supplied at a rate to support a powerful, disruptive discharge. This control action is obtained by the combination of a suitable dropping resistor :and metal at high potential insulated from atmosphere except for a very sharp tip or edge from which a continuous electric charge escapes into atmosphere. As the result, the electric charge from the exposed met-al is drained off so fast as the distance from the sharp tip to ground is reduced that no substantial charge can be built up on the surface of the metal in the gun Where sheltered from the atmosphere by insulation. As is well understood, the metal parts covered with insulation can have a powerful electric charge built up on the outer surface of such metal even though covered with insulation. The amount of electric charge drained off the exposed metal tip is inversely proportional to the clearance between the exposed tip and the nearest grounded item..

Because of the variations in breakdown resistance of air due to humidity, I have found that the value of dropping resistor as hereinbefore set forth will cover most atmospheric conditions. Where a dropping resistor has too high a value, the current drain in a humid atmosphere is so great that the potential drop across the dropping -resistor becomes excessively high and leaves insufficient available potential at the gun for proper operation. This manifests itself by insufficient charging of atomized coating material and, in general, by a reduction in coating eiliciency. Too low a value of dropping resistor can result in a dangerous arc when breakdown occurs. The range specified will cover practical applications from about 2 or 3 inches minimum spacing up to about 18 inches, in which case the working potential will be correspondingly raised.

I claim:

1. An electrostatic gun for spraying coating material comprising:

a body having a forward end provided with a passage therein having an intake for connection to a source of coating material and an outlet through which coatling material is ejected into the atmosphere from the forward end of the body,

means comprising a nozzle for atomizing said coating material on ejection from said outlet,

a valve seat in said passage,

a valve movably mounted in said passage to engage said valve seat for closing said passage,

means applying an electrostatic field to the material passing from the nozzle including an electrical conducting element secured to said valve and extending through said nozzle, and

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

2. An electrostatic gun for spraying coating material in accordance with claim 1 wherein said electrical conducting element terminates in a sharp tip.

3. An electrostatic gun for spraying coating material as specified in claim 1 wherein said electrical conducting element includes said nozzle.

4. An electrostatic gun for spraying coating material as specified in claim 1 wherein said valve seat is adjacent to said nozzle in s-aid passage.

5. An electrostatic gun for spraying coating material comprising:

a body having a forward end provided with a first passage therein having an intake for connection to a source of coating material and an outlet through which coating material is ejected into the atmosphere from the forward end of the body and with a `second passage therein having an intake for connection to a source of air under pressure and an outlet that surrounds and converges toward said outlet of said first passage to eject a ow of air into the atmosphere that converges with coating material ejected by the outlet of said first passage,

means comprising a nozzle for atomizing said coating material on ejection from said outlet of said first passage,

means applying an electrostatic field to the material passing from the nozzle including an electrical conducting element extending through said nozzle, and

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

6. An electrostatic gun for spraying coating material comprising:

a body having a forward end provided with a rst passage therein having an intake for connection to a source of coating material and an outlet through which coating material is ejected into the atmosphere from the forward end of the body and with a second passage therein having an intake for connection to a source of air under pressure and an outlet that surrounds and converges toward said outlet of s-aid first passage to eject a flow of air into the atmosphere that converges with coating material ejected by the outlet of said rst passage,

means comprising a nozzle for atomizing said coating material on ejection from said outlet of said first passage,

a valve seat adjacent said nozzle in said first passage,

a valve movably mounted in said first passage to engage said valve seat for closing said outlet,

means applying an electrostatic eld to the material passing from the nozzle including an electrical conducting element secured to said valve and extending through said nozzle, and

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

No references cited.

M. HENSON WOOD, IR., Primary Examiner.

R. S. STROBEL, Assistant Examiner.

Claims (1)

1. 5. AN ELECTROSTATIC GUN FOR SPRAYING COATING MATERIAL COMPRISING: A BODY HAVING A FORWARD END PROVIDED WITH A FIRST PASSAGE THEREIN HAVING AN INTAKE FOR CONNECTION TO A SOURCE OF COATING MATERIAL AND AN OUTLET THROUGH WHICH COATING MATERIAL IS EJECTED INTO THE ATMOSPHERE FROM THE FORWARD END OF THE BODY AND WITH A SECOND PASSAGE THEREIN HAVING AN INTAKE FOR CONNECTION TO A SOURCE OF AIR UNDER PRESSURE AND AN OUTLET THAT SURROUNDS AND CONVERGES TOWARD SAID OUTLET OF SAID FIRST PASSAGE TO EJECT A FLOW OF AIR INTO THE ATMOSPHERE THAT CONVERGES WITH COATING MATERIAL EJECTED BY THE OUTLET OF SAID FIRST PASSAGE, MEANS COMPRISING A NOZZLE FOR ATMOMIZING SAID COATING MATERIAL ON EJECTION FROM SAID OUTLET OF SAID FIRST PASSAGE, MEANS APPLYING AN ELECTROSTATIC FIELD TO THE MATERIAL PASSING FROM THE NOZZLE INCLUDING THROUGH SAID NOZZLE, AND DUCTING ELEMENT EXTENDING THROUGH SAID NOZZLE, AND MEANS FOR CHARGING SAID ELECTRICAL CONDUCTING ELEMENT TO A HIGH POTENTIAL WITH RESPECT TO WORK TO BE COATED.

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