US2890388A - Electrostatic spray charger - Google Patents

Description

June 9 1959 F. A. cRosKEY ETAL 2,890,338

hlECTROSTATIC SPRAY CHARGER Filed Nov. 30, 1955 3 Sheets-Sheet 2 l ATTORNEY f June 9, 19.59 F. A. cRosKEY Erm.' 2,890,383

ELECTROSTATIC SPRAY CHARGER Filed Nov. A250, 1955 3 Sheets-Sheetl 5 ATTORNEY United States Patna ELECTROSTATIC SPRAY CHARGER Frank A. Croskey, New Baltimore, and `Charles Derwood Tuttle, Wyandotte, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application November 30, 1955, Serial No. 549,983

Claims. (Cl. 317-3) 'Ihis invention relates to electrostatic charging apparatus for charging particles of coating material. More particularly, the invention relates to charging apparatus of the type wherein particles of coating material are projected in atomized form from a spray gun or distributing agency toward the article to be coated through a high density corona discharge zone formed between a discharge electroderand a current collector electrode, both of which are located between the gun and the article to be coated.

The invention is illustrated inan electrostatic spray painting installation in which the charging electrodes are constituted by a plurality of attenuated discharge electrodes, the tips of which are spaced `from an oppositely charged collector electrode with the atomized spray from the spray gun passing through the space between the tips of the ionizing discharge electrodes and the collector electrode toward a grounded workpiece.

The invention has among its general objects to effect improvements in such charging apparatus notably from the standpoint of reducing sparking and other hazards incident to the operation of such devices while increasing their charging eliiciency.

Another object is to provide an improved charging apparatus which aids to decrease paint yconsumption in electrostatic spray painting -installations of this character.

A speciiic object is to provide an improved gin and charging apparatus that reduces electrical leakage from the charging electrode to the gun, thereby reducing the output requirements of the power supply.

Another object is to provide an improved gun and charging electrode assembly by means of which a uniform charging current distribution pattern may be obtained between the discharge electrode and collector electrode of the charging system.

Another object is to provide an improved gun and charger construction which permits the gun to be closely located without fear of sparking to the oppositely charged discharge electrode in order to obtain in the vicinity of the discharge electrodes a spray .pattern of minimum spread and lateral dispersion so that substantially all of the spray passing through the charging zone will oe charged with little or no deposition of paint on the charging electrodes.

Still another object `is to provide an improved mechanical mount for an electrostatic charging apparatus of the above character which is adapted to hold the gun and the charger elements in positive positional alignment and facilitates ready assembly and disassembly of the parts thereof to facilitate cleaning.

The above and other objects, together with the features and advantages attending the present invention, will appear more Yfully lfrom the following detailed description and drawings wherein:

Fig. 1 lis a top plan view with parts broken away of an electrostatic charging apparatus in accordance with the present invention;

l'atented June 9, 1h59 Fig. 2 is a front view taken in the direction 2 2 of Fig. l;

Fig. 3 is an enlarged view with parts broken away of a discharge electrode arrangement in the charging apparatus of the present invention;

Fig. 4 is a side elevation view with parts broken away taken in the direction i-d of Fig. l;

Fig. 5 is a detail enlarged View similar to Fig. l with parts broken away and in section;

Fig. 6 is a view taken in the direction of a part of the apparatus of Fig. l; and

Pig. 7 is a schematic sketch showing the general or ganization of an electrostatic spray painting installation in which the electrostatic charging apparatus of the present invention may be advantageously employed.

Referring to the drawings, Fig. l illustrates an electrostatic charging apparatus in accordance with the present invention which includes a support mount it) for a pneumatic spray gun l2. and a charging electrode assembly constituted by a collector plate ld and a discharge electrode assembly 16. The mount i9 is a onepiece, light metal casting formed by a forwardly extending, vertically disposed platform portion 2d having a laterally offset lobe portion 22, at the rear thereof and a laterally extending yoke portion 24 intermediate its ends and located on the side opposite lthe lobe portion. The lobe 22 has an opening 26 extending therethrough and a cooperating clamp screw 28 by means of which the support mount lil and charger assembly may be mounted on a support standard Sil that may be insulated from ground.

The yoke portion 24 includes a pair ol' curved arms 32, 34 which are joined by a forwardly extending bored boss 36 that supports the discharge electrode assembly 16. The collector electrode l is detachably mounted through a spacer block 35 to the forward end of the platform 20. The side of the platform 2c opposite the lobe 22 receives a laterally extending stud dit to which the gun is secured and mounted, as shown in Figs. l and 2.

The spray gun l2 may be a standard automatic spray gun, such as a Binks Model No. 2l spray gun having a paint inlet 44, an atomizing air inlet 45 and a high-presn sure air actuating inlet 46, and passes through the opening defined by the curved arms of the yoke portion and the platform portion of the support mount. The gun has a vertically offset portion shown at d8 having an opening therein which receives the end of the stud 4t) -for mounting the gun therefrom. The forward end of the gun projects beyond the forward end of the boss 36 and the platform 2t) and is shown in Fig. 5 provided with an air cap 54% and with a fluid tip 56 located centrally of the cap. The cap is provided with a pair of ears 58, 58 which extend beyond the fluid tip 56 and are disposed in a horizontal plane when the support mount 10 and charging electrode assembly are disposed in the position described. The horizontally disposed ears of the cap are provided with air openings as 59 in their adjacent Ifaces in known manner in order to flatten the spray from the gun tip and confine it to a vertical plane or generally vertical direction passing between the tips of the discharge electrodes and the surface of the collector electrode.

The collector electrode 14 is formed of electrically conductive material and is shown as a generally flat plate 60 of broad expanse with a smoothly curved forward end 62 and curved sides 64, 66 defining a shallow pan, the bottom of which-formed by the plate 60-has a rectangular cutout near the open back end of the pan to form a laterally extending ange 68. The llange 68 abuts against the face of the spacer block 38 which is attached to the forward necesa face of the support mount platform by the screws ly 70. The .-v has a notch therein which receives the body of a threaded clamp screw 72 by means of `which the collector plate may be readily attached to or removed from the support mount. The side 66 of the pan is inclined to the side 64, imparting a generally trapezoidal shape to the pan, as viewed in Fig. 4, so that when the collector electrode is mounted in a vertical direction, the inclined side 66 will form a drip trough for any paint or liquid that may fall on the collector plate and flow around the curved sides and the end thereof.

The discharge electrode assembly includes a plurality of elongated, metallic, needle-like rods 85) which are disposed in a radiating or diverging fan-shaped pattern in the direction of the collector plate. The tips of the needles are sharply pointed and are contained in a vertical plane which is normal to the axis of the spray gun and is displaced slightly forwardly of the forward end of the collector plate. The needles are interconnected at their converging ends by a rod 8?. having a rearwardly extending banana-jack or socket M connected thereo. The converging ends of the needles, the rod 82 and the tip of the jack 84 are cast as a unit and embedded in a sectorshaped support head 86 formed of a suitable paint and solvent resistant plastic material, such as an epoxy resin with a small amount of Thiokol plasticizer to improve the hardness and impact qualities thereof.

The head 86 is cemented to an elongated, tubular support rod 88, which may be formed of nylon or the like, and is received in the bored boss 36 of the support mount yoke 24. The back face of the boss portion 36 has a connector 90 attached thereto which receives a coaxial cable 91 from a high voltage power supply 92. The central conductor 93 of the coaxial cable 91 extends through the bore of the rod 88 and has a banana-plug 94 at the end thereof received in the jack 84 to supply the necessary high voltage to the discharge needles. The back end of the tubular support rod 88 emerging rearwardly from the boss 36 is slotted as shown at 98 and terminates ush with the end of an L-shaped washer shown at 100. A spring clip shown at 102 in Fig. 6 surrounds the washer 100 and engages the slot 98 for detachably securing the rod 88 against the back face of the boss.

As best shown in Fig. 3, the discharge needles are arranged in a radiating or fan-shaped pattern with the tips of the needles disposed on an arc so that the inner needles, which are closest to the horizontal plane of symmetry passing through the central needle, are closer to the surface of the collector electrode than the outer needles. This orientation and disposition of discharge needles present a marked advantage over an arrangement in which the needle tips lie on a straight line parallel to the surface of the collector plate. With the tips of the needles arranged on a straight line, it has been found that substan tially all of the total charging current passes through the outermost needles and that only a small portion of the total charging current ows through the inner needles. For all practical purposes, this arrangement utilizes only the extreme outermost needles for charging the paint spray and creates a non-uniform current distribution pattern from the individual needles to the collector electrode.

By disposing the tips of the needles on an arc, or otherwise displacing the outermost needles so that they will be spaced a greater distance from the collector electrode than the inner needles, the current iiow from each needle to the plate will be equalized, and each needle will receive its proportional share of the total current to produce a more nearly uniform concentration of charge throughout the extent of the charging zone.

The collector electrode is maintained at or near ground potential and the discharge needles are maintained at a high negative potential in the neighborhood of, say, around 40 kilovolts with respect thereto in order to induce from the tips of the needles a high density, non-disruptivc, convective, corona discharge which extends principally in a direction approximately transverse to the direction of the advancing spray from the gun. This high density, ionic discharge is of the same polarity as the discharge electrode and is accompanied by a high velocity, silent electrical wind directed substantially at right angles to the direction of travel of the atomized particles from the spray gun to cause the highly charged ionized air particles to collide with the atomized paint particles with maximum impact, thereby placing a very effective surface charge of static electricity on the atomized paint particles. For optimum charging efficiency it is desirable that the potential gradient between the tips of the discharge electrodes and the collector electrode be at least, and preferably in excess of, l0 kilovolts per inch.

The support mount 10, gun 12 and collector electrode 14 are electrically interconnected and may be connected over conductor 106 from the ground insulated support stand 30 to an output terminal of the power supply 92, which may be of the spark anticipating or inverse current regulating variety such as is disclosed in British Patent 673,410 of common ownership herewith. The high potential side or terminal of the power supply is connected over coaxial cable 91 to the discharge elecn trodes 16, as described above. The discharge electrodecollector, electrode current is returned to ground through a regulating resistance contained in this power supply and develops an automatic regulating voltage thereacross a few volts removed from ground that controls the output voltage from the power supply, causing interruption or appreciable reduction thereof if the discharge electrodecollector, electrode current should attempt to increase for any reason and otherwise cause a spark or flashover. The charging apparatus can, of course, be employed with other forms of power supplies, and in the absence therein of a spark anticipating arrangement of the above character, the gun and collector electrode could be directly grounded.

The low voltage input terminals of the power supply 92 are connected over conductors 108, 110 through the nor,- mally open contacts 112, 114, of a relay-actuated circuit breaker 111 to a low voltage source of power. The operating coil 115 of the relay 111 is connected for enen gization from the low voltage secondary side 118 of a step-down transformer 116 whose primary winding 117 is connected for energization from the line side of the breaker contacts. Connected in series with the secondary winding 118 of the transformer is a pair of normally-open contacts 120 of a flow-actuated switch 122 which is connected in an air line 124 from a source of high pressure air 126 for actuating the needle 130 in the uid inlet valve of the spray gun. Retraction of the needle 130 from the opening 131 in the fluid tip 556 of the gun nozzle admits iluid under slight pressure from the paint reservoir 132 which is connected by paint line 134i` to the paint line of the spray gun. An atomizng air source is shown at 136 connected over line 138 to the atomizing air inlet 45 of the spray gun.

The atomized spray from the gun passes between the tips of the discharge needles and the surface of the col.- lector electrode and through the corona discharge zone therebetween where it becomes charged and is caused to be attracted and deposited on the oppositely charged, grounded Workpieces 142 as they are carried on a grounded conveyor 144 past the spray gun.

In order to prevent the high density corona discharge from the discharge needles from spilling over into the spark discharge region and causing a spark from thc needles to the forward end of 'the grounded spray gun, the cap 54 and the duid insert or tip 56 of the spray gun 12 are, in accordance with one aspect of the present invention, formed of electrically non-conductive material, such as nylon for example. The needle 130 also could be formed of nylon, if desired. This expedient has been found to substantially eliminate any tendency of sparkover from the discharge needles to the spraygun, which is an important consideration from the standpoint of making such equipment safe from fire hazards and for operating personnel. Except for their material composition, the cap 54, uid insert 56 and paint control needle 130 may be the same shape and configuration as the corresponding parts normally supplied with the spray gun.

The use of the nylon cap and iluid tip insert also permits the spray gun to be located more closely to the discharge needles 80 Without fear of sparking than a spray gun with a metal cap and tip. The nylon cap and uid insert in etect, permit the discharge needles to be located closer to the gun and thus indirectly enable keeping the needles and the entire structure somewhat cleaner. This expedient also enables the charging of a greater portion of spray pattern from the spray gun than a metalcapped spray gun which, under the same operating conditions, must be located a greater distance behind the discharge needles in order to prevent spark hazards and will, therefore, have a greater spread in its spray pattern in the plane of the discharge needles.

In a related aspect it has been found that the use of a nylon cap and luid insert tip for the previously employed metal cap and tip of the spray gun also reduces current leakage from the inner needles to the ears and/ or to the forward end of the gun, thus reducing, to some extent, the power requirements and capacity of the power supply. The current leakage from the needles to a gun with a metal cap will result in weakening or making a smaller portion of the total current available from the inner needles for charging the paint particles, thus reducing the charging eiciency of the system and create a non-uniform current distribution pattern from the inner needles to the collector plate. The use of a nylon cap and tip eliminates the current leakage from the inner needles and assures that substantially all of the current will be available and most electively utilized to charge the paint particles. In this aspect, the nylon cap and fluid insert thus cooperate with the fan-shaped, radiating array of arcuately disposed needles to provide a uniform current distribution pattern from each needle to the collector plate and assures high charging efiiciency and the most effective paint utilization in a system of this character.

What is claimed is:

1. Apparatus for charging a liquid spray including a spray nozzle having a spray cap issuing liquid spray therefrom, an ionizing electrode and a collector electrode spaced from said ionizing electrode, both of said electrodes being spaced from said spray nozzle in the region of the advancing spray therefrom, and a high potential power supply maintaining said ionizing electrode at a potential dilerent from that of said collector electrode and said spray nozzle, said spray cap being composed of electrically non-conducting material, said ionizing electrode comprising an array of attenuated needles directed toward said collector electrode and disposed in a plane transverse to the direction of the spray from said spray nozzle Awith the spray passing in the space between the tips of the needles and the collector electrode, the tips of the outermost needles of said array being spaced a greater distance from said collector electrode than the tips of the inner needles between said outermost needles.

2. Apparatus for charging a liquid spray including a spray nozzle having a spray cap issuing liquid spray therefrom, an ionizing electrode and a collector electrode spaced from said ionizing electrode, both of said electrodes being spaced from said spray nozzle in the region of the advancing spray therefrom, and a high potential power supply maintaining said ionizing electrode at a potential different from that of said collector electrode and said spray nozzle, said spray cap being composed of electrically non-conducting material, said ionizing electrode comprising an array of attenuated needles directed toward said collector electrode and disposed in a plane transverse to the direction of the spray from said spray nozzle, the tips of said needles being disposed on an arcuate line so that the inner needles of said array will be closer to said collector electrode than the outermost needles.

3. Apparatus for charging a liquid spray including a spray nozzle having a spray cap issuing liquid spray therefrom, an ionizing electrode and a collector electrode spaced from said ionizing electrode, both of said electrodes being spaced from said spray nozzle in the region of the advancing spray therefrom, and a high potential power supply maintaining said ionizing electrode at a potential different from that of said collector electrode and said spray nozzle, said spray cap being composed of electrically non-conducting material, said ionizing electrode comprising a fan-shaped radiating array of attenuated needles diverging in the direction of said collectoielectrode and disposed in a plane transverse to the direction of the spray from said spray nozzle, the ltips of said needles being disposed on an arcuate line so that the inner needles of said array will be closer to said collector electrode than the outermost needles.

4. Apparatus for charging a liquid spray including a spray nozzle having a spray cap issuing liquid spray therefrom, an ionizing electrode and a collector electrode spaced from said ionizing electrode, both of said electrodes being spaced from said spray nozzle in the region of the advancing spray therefrom, and a high potential power supply maintaining said ionizing electrode at a potential different from that of said collector electrode and said spray nozzle, said spray cap being composed of electrically non-conducting material, said ionizing electrode comprising an array of attenuated needles disposed in a plane transverse to the direction of the spray from said spray nozzle but out of the path thereof, all the needles which form said ionizing electrode being arranged as the ribs of a fan.

5. In an electrostatic spray coating installation for spraying a desired pattern of atomized material, the combination of a spray gun having a spray cap and projecting atomized particles of coating material, means enhancing the formation of said pattern by said` particles including discharge electrode means comprising an array of pointed members, all the pointed members 'which form said discharge electrode means being arranged as the ribs of a fan and the base of all pointed members lying in the same plane, and current collector electrode means on opposite sides of the path of said particles projected from said spray gun and a high potential power supply maintaining said discharge electrode means at a high absolute potential relative to said current collector electrode means and said spray gun, said spray cap being composed of electrically non-conducting material such as nylon permitting such spray gun to be located closer to the oppositely charged discharge electrode without sparking.

References Cited in the file of this patent UNITED STATES PATENTS 1,346,811 Diebold July 20, 1920 2,525,347 Gilman ..-00L 10, 1950 2,710,773 Sedlacsik June 14, 1955

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