US3251551A - Electrostatic coating system - Google Patents

Description

May 17, 1966 A. c. WALBERG ELECTROSTATIC COATING SYSTEM Original Filed July 19, 1962 r QM am Mm @N INVENTOIQ United States Patent 3,251,551 ELECTROSTATIC COATING SYSTEM Arvid C. Walherg, Lombard, Ill., assignor to H. G. Fischer & Company, a corporation of Illinois Continuation of application Ser. No. 211,006, July 19, 1962. This application Jan. 19, 1966, Ser. No. 515,682 12 Claims. (Cl. 239-15) This invention relates to an electrostatic coating system and is a continuation of my copending 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 application, a sharp edge or sharp tip of metal charged to a high potential and disposed in the gun outlet region serves to define an intense electrostatic field normally extending toward the work to 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 flashover. A fiashover in a conventional system will generally result in an are having suflicient body or energy to start a fire or to be dangerous. In a system embodying the present invention, any flashover 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 other plastic having sufficient dielectric value to prevent any significant 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 fine 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 cases, 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 out above, a dropping resistor whose value is determined by the normal operating potentialof the system with the gun. By normal operating potential is meant the potential causing a current drain of about .1 milliampere to flow. A substantial portion of this current drain is the current'discharged into atmosphere by the sharp discharge tip of the gun. 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.

p In a system utilizing the invention, the spacing between, 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 Patented May 17, 1966 potentials for operating systems of this character will range 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 sufficiently low paint resistance to permit the paint to readily become charged electrostatically.

Between electrodes, each having an area of three square inches and having a separation between electrodes of one quarter of an inch, the direct current resistance of the Some current will flow back through the paint coating material is preferably adjusted to lie between one and two megohms, although up to five 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 resistance 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 range will be zero to 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.

If the coating material has too low a resistance 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.

In general, the determination of which thinner to use 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.

For a full description of the invention, reference will now be made to the drawing wherein the single figure is in a view of a system embodying the present invention.

The gun shown in the figure may be of the hand or automatic type. The gun may also be of the type wherein coating material is discharged in 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. types of gun, atomization is principally mechanical.

Inasmuch as guns of the above types are well known, no detailed structural description thereof is deemed to be necessary except for 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 In either of the above trated there has barrel carrying nozzle assembly generally indicated by 12. Barrel 10 and nozzle assembly 12 has the outside thereof of electrically insulating material. Within the interior of barrel 10 and nozzle assembly 12 is a suitable means for discharging air and coating material in the region generally indicated by 13. Horns 14 and 15 laterally offset from the axis of the gun barrel have air jets for 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 13. 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 metal edge or tip 16 which projects into the air a short distance and which has minimum surface area exposed to the air, allother metal within nozzle cap 12 and barrel 10 will be adequately insulated from air to prevent any substantial electrical discharge from 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 off. 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. This dropping resistor is provided with suitable insulation around the outside thereof. One resistor terminal is connected to the metal within nozzle assembly 12 by 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 16 of the gun and the adjacent dropping resistor terminal is reduced to a minimum. Furthermore, the short length of wire 24 makes it unnecessary to provide any ground shielding. Thus the electrical capacitance of the part of the system including charged tip 16 and droping resistor 23 on the one hand and ground on the other hand is reduced to a negligible value. In 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 flexible cable used in this industry and is usually in the form of a coaxial cable from about 20 feet to as much as 50 feet in length having or A 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 fact 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 regard 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. As an example, a floor will have sufiicient 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 heavy insulation for wire 24 or, if desired, a separate clamp may be provided.

Gun 20 will be provided with air and coating material inlets. The entire gun including barrel 1t 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 a have one terminal grounded and the high potential terminal connected to the central conductor of the coaxial cable. Power supply 3% provides a high direct potential, the value being as previously indicated. The power supply includes suitable means for controlling the output potential and current, as well as safety means for 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 energized from a conventional alternating current power line.

Insofar as coating material is concerned, gun 20 will have flexible hose 32 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 for air and is supported by clamp 31a.

The value of resistor 23 will depend upon the desired normal operating conditions of the system. Apart from any particular value of resistor 23, its location in combination with an electrically insulated gun having minimum exposed metal at high potential imparts improved operating characteristics to an electrostatic coating system.

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 metal. 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 carbide 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 metal 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 ofit 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 insuflicient 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 efficiency. Too low a value of dropping resistor can result in a dangerous are When breakdown occurs. The range specified will cover practical application 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 liquid coating material comprising a body provided with a passage having an intake for connection to a source of liquid coating material, said passage having an outlet, a nozzle assembly including a nozzle secured to said body at said outlet through which'liquid coating material can be discharged into atmosphere, means for mechanically atomizing said coating material on discharge from said nozzle, said body including electrical insulation covering all metal in said gun which is maintained at a high potential, said electrical insulation having sufiicient thickness for withstanding electrical potentials used in electrostatic coating, a single metallic charging electrode extending forwardly from the interior of the gun and terminating in an exposed pin having a single sharp point and means for charging said electrode to a high potential with respect to work to be coated, said charging means includ ing a mass of metal at high potential disposed in proximity to said nozzle, said mass of metal having substantial capacitance, and a connection to one terminal of a high potential source, the other terminal of said high potential source being connected to said work.

2. 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 coating 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 shield of electrical non-conductor material surrounding all conducting portions of said body,

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

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

3. An electrostatic gun for spray coating material as specified in claim 2, wherein said means applying an electrostatic field to the materials passing from the nozzle terminates in a sharp projection.

4. An electrostatic gun for spraying coating material as specified in claim 2, wherein said electrical conducting element tapers to a sharp exposed projection forming an acute included angle.

5. An electrostatic gun for spraying coating material as specified in claim 2, wherein said electrical conducting element extends through said shield at said nozzle to a sharp tip, whose cross section forms an acute angle.

6. An electrostatic gun for spraying coating material as specified in claim 2, wherein said electrical conducting element extends forwardly through said outlet.

7. An electrostatic gun for spraying coating material as specified in claim 2, wherein said means applying an electrostatic field to the material passing from the nozzle includes said nozzle, said nozzle extending through said shield.

8. An electrostatic gun for spraying coating material as specified in claim 2, wherein said electrical conducting element terminates in an exposed sharp annular edge.

9. An electrostatic gun for spraying coating material as specified in claim 2, wherein said means applying an electrostatic field to the material passing through the nozzle includes said nozzle and an electrical conducting needle extending forwardly through said nozzle and tapering to a sharp exposed tip.

10. 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 coating material is ejected into the atmosphere from v the forward end of the body,

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

a shield of electrical non-conductive material surrounding all conducting portions on the forwad end of said body,

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

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

11; The gun according to claim 1 wherein said gun' includes means for directing blasts of air from regions laterally offset from the discharge nozzle with the air blasts being directed toward the space in front of the discharge nozzle for effecting air atomization of coating material.

12. The gun according to claim 1 wherein said body includes a gun barrel of electrically insulating material.

References Cited by the Examiner UNITED STATES PATENTS M. HENSON WOOD, 111., Primary Examiner,

R. S. STROBEL, Assistant Examiner.

Claims (1)

1. 2. 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 COATING 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 SHIELD OF ELECTRICAL NON-CONDUCTOR MATERIAL SURROUNDING ALL CONDUCTING PORTIONS OF SAID BODY, MEANS APPLYING AN ELECTROSTATIC FIELD TO THE MATERIAL PASSING FROM THE NOZZLE INCLUDING AN ELECTRICAL CONDUCTING ELEMENT EXTENDING THROUGH SAID SHIELD AT 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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