US3907202A

US3907202A - Spray-gun apparatus for atomizing paint or similar liquids

Info

Publication number: US3907202A
Application number: US468261A
Authority: US
Inventors: Michel Binoche
Original assignee: SKM SA
Current assignee: SKM SA
Priority date: 1973-05-10
Filing date: 1974-05-08
Publication date: 1975-09-23
Anticipated expiration: 1992-09-23
Also published as: DE2422597A1; JPS6012915B2; JPS5053430A; DE2422597B2; NL177663C; NL177663B; NL7406278A; AU6882274A; SE409826B; CA1015382A; GB1461436A; IT1011415B; BR7403837D0

Abstract

The present invention relates to spray-guns apparatus or similar apparatus for forming by pneumatic pressure a fine spray or mist of paint or other similar liquid in the form of a flat, fan-like sheet, usually directed onto a surface, characterised in that the dimensions of the hydrostatic atomizing nozzle and the feed means are calculated for withstanding paint feed pressures of 145 to 1,450 psi (10 to 100 bars) and a compressed-air supply pressure of 7 to 29 psi (0.5 to 2 bars), and that the atomizing head further comprises an annular orifice concentric to said hydrostatic atomizing nozzle and supplied with compressed air under a pressure substantially equal to that of the other orifices, said annular orifice being so disposed as to direct a hollow tapered air jet towards a point close to the outlet orifice of said nozzle.

Description

United States Patent 11 1 Binoche 1451 Sept. 23, 1975 21 App1.No.:468,26l

[30] Foreign Application Priority Data May 10. 1973 France 1. 73.17011 May 10, 1973 France i... 73.17012 [52] U.S. Cl. 239/15; 239/41713; 239/421; 239/4245 [51] Int. Cl.*.... 805B U06; 8058 1/30; B05B-5/02; B05B 7/04 [58] Field of Search 239/3, 15, 398 400, 239/407, 416.5, 417.3, 421-425.5, 428, 432, 597-599 [56] References Cited UNITED STATES PATENTS 3,253,782 5/1966 Fischer et a1. 239/15 3589607 6/1971 Wolf et a1. 239/15 3,630,441 12/1971 Felici et a1. 239/15 3,635,401 1/1972 Bromley et a1. 239/15 3.659.787 5/1972 Kawasaki 239/15 3.670961 6/1972 Tholome 239/15 3,677,470 7/1972 Probst et 1. 239/15 3.693.877 9/1972 c6wan. 239/15 3,731,145 5/1973 Senay... .239/15 x 3,737,099 6/1973 Shaffer 239/15 3.746253 7/197'3 Walberg 239/15 Primary ExaminerRobert 5. Ward, Jr.

[57] ABSTRACT The present invention relates to spray-guns apparatus or similar apparatus for forming by pneumatic pressure a fine spray or mist of paint or other similar liquid in the form of a flat, fan-like sheet, usually directed onto a surface, characterised in that the dimensions of the hydrostatic atomizing nozzle and the feed means are calculated for withstanding paint feed pressures of 145 to 1,450 psi (10 to 100 bars) and a compressedair supply pressure of 7 to 29 psi (0.5 to 2 bars), and

that the-..atomizing head further comprises an annular orifice concentric to said hydrostatic atomizing nozzle and supplie'd wit h compressed air under a pressure substantiallyiequal to that of the other orifices, said annular orifice being so disposed as to direct a hollow tapered air jettowards a point close to the outlet orifice of said nozzle.

16 Claims, 10 Drawing Figures US Patent Sept. 23,1975 Sheet 1 of4 3,907,202

lZu b 26 15 13 (L 15 2d 2 1- 28b US Patent Sept. 23,1975 Sheet 2 of4 3,907,202

Sheet 3 0f 4 R a m US Patent Sept. 23,1975

US Patent 'Sept. 23,1975 Sheet4 of4 3,907,202

llllllllnnnn l! ITV SPRAY-GUN APPARATUS FOR ATOlVIIZING PAINT OR SIMILAR LIQUIDS BACKGROUND OF THE INVENTION The present invention relates to spray-gun apparatus or similar apparatus for forming by pneumatic pressure a fine spray or mist of paint or other similar liquid in the form of a flat, fan-like sheet, usually directed on to a surface.

Apparatus of this character are widely known, wherein a cylindrical or tapered jet of liquid paint is broken up as it emerges from the corresponding nozzle by various jets of compressed air or other gas, the effects of these interference jets being devised for converting the liquid atomized jet into a fiat, fan-shaped sheet of paint spray. Now this conventional type of apparatus is characterised not only by a poor power efficiency but also by the fact that it requires very large outputs of compressed air or gas when a relatively large amount of paint has to be sprayed. Therefore, such apparatus involves a high and therefore costly air or gas consumption, and moreover the particles of atomized paint are driven at very high speed by the expanding compressed air, thus causing a considerable dispersion of said paint particles in the surronding atmosphere. As a result, substantial amounts of paint are lost, and the air breathed by the operator is seriously polluted. If a relatively volatile and possibly ignitable paint is used, large amounts of inflammable paint vapours are dispersed by the expanding compressed air or gas, thus creating serious explosion hazards, notably in the presence of electric appliances or machines in operation.

Most of the above-listed inconveniences are usually avoided in modern paint spraying rooms and workshops comprising a spray-gun, either fixed or movable, of which the atomizing head is provided with a nozzle designed for spraying the paint hydrostatically in the form of a flat, fan-shaped paint jet. With these known arrangements, widely spread nowadays, the paint is atomized without using compressed air. However, the hydrostatic atomizing nozzles must comprise very fine orifices in the form of minute slots, but these are on the other hand prone to become clogged by the particles in suspension in the paint material; in addition, these fine slots are subjected to a considerable degree of wear, especially when spraying paints containing abrasive fillers. To avoid this last-mentioned drawback, the use of tungsten carbide hydrostatic spray nozzles has been proposed, but these are not only extremely difficult to manufacture, but also very expensive. Finally, this hydrostatic spray nozzle must be fed with paint under a very high pressure, for example 2,900 psi (200 bars), and obviously this entails a number of serious inconveniences: thus, plants of this type must comprise highpressure pumps, in most cases multi-stage pumps, likely to develop considerable leaks and even cause explosions. Finally, the atmospheric air displacement caused by the high-pressure paint jet is attended by an ample dispersion of fine paint particles in the surrounding atmosphere.

In an attempt to avoid these inconveniences of hydrostatic spray-gun means, two types of equipments have already been proposed in the art:

a. In a first type, a hydrostatic atomizing nozzle is fed with paint under a relatively moderate pressure, notably lower than 67 psi (6 bars), so that said nozzle will form at least one flat, fan-shaped liquid-paint spray;

however, in this case the jet speed in relation to the still surrounding atmosphere is not sufficient for atomizing the paint spontaneously. Therefore, the atomization of this flat jet of liquid paint is obtained by causing an air jet, also of fan-like shape, to impinged thereagainst. Under these conditions, the atomization is of purely pneumatic type and facilitated however by the flat and thin shape of the paint jet. Although apparatus of this first type involve the use of less pressure and compressed air than in conventional and purely pneumatic spray-gun apparatus, mentioned in the above preamble, their efficiency is far from being very satisfactory.

b. In a second known type of paint atomizing apparatus the paint is atomized in a purely hydrostatic manner, i.e. by using a high-pressure paint feed, generally under pressurevalues higher than 1,450 psi bars), but the presence of orifices disposed on either side of the hydrostatic spray nozzle for directing compressed air jets on to the two flat faces of the paint jet permits of using a nozzle having an outlet orifice considerably larger than in apparatus not comprising such auxiliary compressed air jets; however, the force of these lastmentioned jets is not sufficient to enable them to assist in the paint atomizing process, this atomization taking place also in this case spontaneously and in direct contact with the still surrounding atmosphere.

SUMMARY OF THE INVENTION The spray-gun apparatus according to this invention for atomizing paint or a similar liquid comprises likewise a spray-gun, whether fixed or movable, of which the spray head comprises a nozzle for the hydrostatic atomization of paint, adapted to create a flat, fanshaped paint spray or jet, and at least two orifices disposed on either side of said nozzle for directing air jets on to the flat, fan-shaped paint spray or jet, together with means for supplying paint to said nozzle and compressed air to said orifices under suitable pressures, respectively. However, the apparatus according to this invention is so designed that all the inconveniences characterising conventional purely hydrostatic spraygun apparatus of the two types mentioned hereinabove, without resorting to more complicated and costly means.

The paint atomizing apparatus according to this invention is of the type broadly mentioned hereinabove, and characterised in that the dimensions of the hydrostatic atomizing nozzle and the feed means are calculated for withstanding paint feed pressures of to 1,450 psi 10 to 100 bars) and a compressed-air supply pressure of 7 to 29 psi (0.5 to 2 bars), and that atomizing head further comprises an annular orifice concentric to said hydrostatic atomizing nozzle and supplied with compressed air under a pressure substantially equal to that of the other orifices, said annular orifice being 50 disposed as to direct a hollow tapered air jet towards a point close to the outlet orifice of said nozzle.

The apparatus according to the present invention is free of all the inconveniences arising in devices operating under paint feed pressures above 1,450 psi (100 bars), so that lowor medium-pressure pumps can safely be associated therewith, thus reducing considerably the risks of leakage or other disturbances. On the other hand, the pressure and output of the air utilized in the apparatus of this invention are reduced considerably in comparison with those required for operating purely pneumatic apparatus, and this constitutes an advantageous feature from the economical point of view and also in that the amount of possibly inflammable paint and notably paint vapours dispersed in the surrounding atmosphere is reduced considerably. By atomizing the liquid jet of paint by means of the various jets of compressed air as contemplated in the present invention, and under the operating conditions mentioned in the foregoing, it is possible not only to produce a paint mist as fine as that obtained with hitherto known apparatus but also to widen the outlet orifice of the hydrostatic atomizing nozzle, with the dual advantage of reducing its wear and the risk of clogging this orifice.

According to a preferred form of embodiment of the present invention, the nozzle of the atomizing head is fed with liquid paint under a pressure of 145 to 290 psi 10 to 20 bars), and its orifices are supplied with compressed air at a pressure of 22 to 29 psi (L to 2 bars).

By adhering to these preferential operating conditions the speed of the liquid paint jet issuing from the nozzle is so reduced that if no compressed-air jets were applied thereto, said jet would be only very coarsely atomized. However, the action of the compressed air jets is sufficient for producing an extremely fine atomization as desired, by using compressed air jets considerably weaker than those required for atomizing comparably and through purely pneumatic means a liquid paint jet issuing from a large orifice (if compared with the orifices used in purely hydrostatic atomization). Also in comparison with these last-mentioned orifices, the nozzle orifice of the apparatus according to the present invention may be considerably larger, with all the attendant advantages already described hcreinabove. Moreover, at least in the case of paints containing moderately abrasive fillers, this nozzle may be made simply of heat-treated steel, which is obviously easier to ma chine, and more economical, than tungsten carbide.

According to another feature characterising this invention, the pair of additional air jets are deflected by a frustoconical surface before impinging on the flat faces of the paint jet.

' In fact, it is known in the art that high-pressure spray painting plants operating according to the so-called airless" method produce at the nozzle outlet a flat jet in the form of a veil of paint becoming gradually wider and thinner as as the distance from the nozzle towards the area in which it is atomized increases. If a lower paint pressure is used, the edges of this veil are defective and two substantially cylindrical jets depart therefrom. These jets are not properly atomized and produce paint deposits clearly spaced from the central main jet.

Now it was observed that these drawbacks could be avoided by using air jets deflected by a frustoconical surface. The paint pressure (considering the specific case of an ordinary glycerol-phthalic paint at thirty'seconds with a Ford No. 4 cut, may be selected from the range of 145 to 290 psi to bars). The pressure of the additional airjets delivered through a pair of side orifices having a diameter of one millimeter may be of the order of 14.5 to 29 psi l to 2 bars) and the central tapered jet is fed under a considerably lower pressure.

The paint and airjet assembly has a very low inherent power, less than that of a conventional airless type jet; as a result, paint mist dispersions due to rebound on painted surfaces are minimized.

This spray-gun apparatus may also and advantageously be utilized for atomizing paint hydrostatically.

In existing apparatus of this type equipped with hydrostatic atomizing nozzles, the velocity of the jet of liquid paint directed through the narrow nozzle orifice is so high that the particles of atomized paint escape to a large extent the action of the directing electrostatic field prevailing between one or a plurality of electrodes equipping the atomizing head of the apparatus and the nearest objects, at ground potential, notably the articles or pieces to be painted.

The means for hydrostatically spraying paint or similar liquids according to the present invention further comprises a nozzle of electrically conducting material and means for applying to said conducting nozzle an electric direct current of high value in relation to the ground and to the articles and pieces to be painted.

Insofar as the atomization of the liquid paint jet issuing from the nozzle of the apparatus according to the present invention is due primarily to the interaction between the liquid paint jet and the various jets of compressed air, the various parameters of the equipment according to this invention can easily be so selected that the initial velocity of the particles of atomized paint be low enough to cause nearly the whole of these particles to be exposed to the directing action of the electric field created by the electrically charged elements of the atomizing head, notably the nozzle for hydrostatically atomizing the paint. In this respect, the apparatus according to this invention, though comprising a nozzle for hydrostatically atomizing paint, offers the same advantages as a purely pneumatic apparatus while having a considerably lower air consumption.

In a particularly advantageous form of embodiment of the apparatus according to this invention for the hydrostatic atomization of paint, the atomizing head further comprises at least one primary needle-type electrode brought to the same electric voltage as the electrostatic atomizing nozzle and disposed outside the flat jet of paint, so that the point of this electrode be adjacent one flat side or face of said jet, at the level of the paint atomization front, or alternatively slightly downstream of this atomization front, and also at least one secondary insulated electrode having a first blunt end registering with the point of the primary electrode, on the opposite side of said flat jet, outside but relatively close to said flat jet, and another pointed end located at a relatively great distance from the paint jet, and so arranged that it projects preferably from the front of the spray-gun, towards the objects or piece to be painted. In this case, the point of the primary electrode and the first end of the secondary electrode are preferably aligned normally to the flat paint jet. This electrode arrangement is particularly advantageous in that it will cfficiently charge the sheet of atomized paint, thus reducing the percentage of paint particles escaping from the directing electric field, which are usually dispersed and lost in conventional apparatus, while positively preventing the tips of the various electrodes from becoming gradually fouled by the paint and thus lossing their electrical efficiency prematurely with time.

An apparatus for the electrostatic spraying of paint is already known wherein the atomizing head comprises a hydrostatic atomizing nozzle together with a primary charged electrode and a secondary insulated electrode, both electrodes being aligned normally to the flat sheet of paint. However, in this known arrangement the aligned points or tips of the two electrodes are located well upstream to the paint atomizing front. Now, it was found that the electrical efficiency of these electrodes increases at their distance from the likewise charged hydrostatic atomizing nozzle increases, probably because the electric field of this nozzle tends to disturb the electric field of the other electrodes and, on the other hand, the particles of atomized paint tend to foul rapidly the points or tips of said other electrodes if they are located well downstream of the paint atomizing front. Therefore, according to this invention, the ends or tips of the primary and secondary electrodes are disposed preferably at the level of the atomization front, or slightly downstream thereof, but close enough to prevent the tips of these electrodes from being fouled too rapidly by the atomized paint.

BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the paint spraying apparatus according to this invention will appear as the following description proceeds with reference to the attached drawings illustrating diagrammatically by way of example typical forms of embodiment of the invention, given by way of illustration, not of limitation. In the drawings:

FIG. 1 is a side elevational view of a first form of embodiment of the spray-gun of this invention, with a fragmentary vertical section showing details of its atomizing head;

FIG. 2 is a sectional view showing on a larger scale the atomizing head of the spray-gun of FIG. 1;

FIG. 3 is a section taken along the line IIIIII of FIG.

FIGS. 4 and 5 illustrate on a larger scale two different forms of embodiment of the atomizing head of the spray-gun in an apparatus according to this invention;

FIG. 6 is a sectional view showing on a larger scale details of the head shown in FIG. 2;

FIGS. 7 and 8 are two different forms of embodiment of the atomizing head;

FIG. 9 is an elevational view of a spray-gun for electrostatically spraying paint, the head thereof being shown in fragmentary section, and

FIG. 10 is a sectional view showing on a larger scale a modified form of embodiment of the atomizing head of the spray-gun of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENT The paint atomizing spray-gun apparatus illustrated diagrammatically in FIG. 1 comprises essentially a grip 1, a body 2 and an atomizing head 3.

The front portion of body 2 (left-hand portion of FIG 1) has a chamber 4 formed therein which communicates with the surrounding atmosphere via a union 5 adapted to be connected preferably by means of a flexible hose (not shown) to a paint feed pump 6 of the lowor medium-pressure type, the suction side of this pump 6 being connected in turn through another hose to a paint reservoir or can 7. In the outermost left-hand portion, as seen in FIG. 1, of the chamber 4 of body 2, which is formed with inner threads, a socket 8 having a coaxial cylindrical bore 8a is screwed home; at its left-hand end (FIGS. 1 and 2) this bore 80 has an extension 8b of smaller diameter, the shoulder formed be tween these bores 8a and 8b providing a seat engageable by a needle valve or like valve member 9 carried by the end of a rod 10 mounted for axial sliding movement in said bore 8a. The right-hand end of rod 10 emerging in a fluid-tight manner from the body 2 is operatively connected to a trigger l1 fulcrumed at 11a to the spray-gun body 2. A spring (not shown) constantly urges the rod 10 operatively connected to the trigger 11 towards the left, as seen in FIG. 1, so as to seat the valve member 9 and thus close the minor bore 8b. the

latter has an axial forward extension 12a formed in a ring 12 having its rear side (i.e. the right-hand end face in FIG. 2) urged against the front face of socket 8 and its front side (i.e. the left-hand end face in FIG. 2) engaged by a nozzle 13 for hydrostatically spraying or atomizing paint, and shown in plain view in FIGS. 1 and 2. This nozzle 13 has an axial passage formed therethrough, in alignment with

bores

12a and 8b mentioned hereinabove, and this passage opens at the front of said nozzle into an atomizing slot 13a which, in the specific form of embodiment contemplated herein, extends at right angles to the plane of FIGS. 1 and 2. The design of this nozzle 13 and notably of its hydrostatic atomizing slot 13a differs only slightly from that of nozzles currently used up to now in purely hydrostatic sprayguns. However, instead of consisting of tungsten carbide, the nozzle 13 of this invention may consists simply of heat-treated steel, at least insofar as the spraygun contemplated is intended for spraying paints containing only moderately abrasive fillers. On the other hand, the front face of nozzle 13, in which the atomizing slot is formed, has preferably a part-spherical configuration merging into a frustoconical lateral surface.

The ring 12 and nozzle 13 are furthermore surrounded by an annular member 14 having a frustoconical front face constituting the lateral extension of the front tapered face of said nozzle 13. Finally, this nozzle 13 is locked against angular movement in the aforsaid member 14 by suitable means (not shown).

The front portion (i.e. on the left as seen in FIG. 2) of the tubular socket 8 is also surrounded by a shouldered ring 15 also screwed in the inner threads of the front bore or chamber 4 of body 2, ahead of the screw threaded portion of socket 8. The inner diameter of ring 15 is such that is provides around the front end of said socket 8 an annular chamber 15a communicating through a passage 16 with a duct 17 formed in the upper portion of the spray-gun body 2, this duct 17 communicating in turn via other passages (not shown) with a compressed-air supply union 18. A valve 33 adapted to actuated by means of the trigger 11 is inserted in the compressed-air passages. In the form of embodiment illustrated the union 18 is connected via a' flexible hose (not shown) to a source of compressed air shown only diagrammatically at 19. The duct 17 opens on the other hand, at the front end of said body 2, into a smaller bore 17a, thus forming with the duct 17 a seat engageable by a needle valve 20a carried by the front end of a rod 20 slidably and coaxially mounted in the duct 17, and operatively connected to an adjustment knob 20b.

An air head 21 is fitted over the nozzle 13 and annular member 14, and locked in relation thereto by a nut 22 formed with inner threads engaging corresponding screw-threads 23 formed on the front end of said body 2. The air head 21 comprises a central aperture engaged by the

members

13 and 14; this aperture comprises a frustoconical inner surface so disposed as to form in conjunction with the frustoconical surfaces of said

members

13 and 14 an annular passage 23 of which the mean diameter decreases in the forward direction, i.e., to the left as seen in FIG. 2.

Longitudinal grooves 24 machined at spaced angular intervals (FIG. 3) in the outer cylindrical surface of member 14 open into the rear end of an annular chamber 25 (at the right-hand end of member 14, as shown in FIG. 2). This annular chamber 25 communicates directly with the annular chamber 15a already mentioned in the foregoing. A seal 26 capable of withstanding the pressure of compressed air is disposed between a shoulder of ring 15 and the rear portion of air head 21, the latter being slipped to its final position over the front end of said ring 15. On the other hand, the annular member 14 is locked against movement about its axis in relation to said air head 21 by a longitudinal key 27 so that the slot 13a of nozzle 13 (also locked angularly in relation to the annular member 14) be disposed at right angles to the plane of FIG. 2, as mentioned hereinabove.

The front face of the air head 21 comprises a frusto conical surface 21a converging towards an apex located slightly ahead of the atomizing slot 13a. A pair of substantially circular-sectioned

passages

28a and 28b which, in this specific form of embodiment, extend substantially at right angles to, and on either side of said atomizing slot 13a, open towards the base of said frustoconical surface 21a, as shown, notably in FIG. 2. These

passages

28a and 28b are fed with compressed air via

longitudinal passages

29a and 29b, respectively, from an annular chamber 30 formed in said nut 22 between the front face of body 2 and the rear face of air head 21.

The paint spray-gun according to this invention, as shown diagrammatically in FIG. 1 and described hereinabove is operated as follows: the paint pump 6 is selected and adjusted to deliver paint to the union of the spray-gun under a pressure of 145 to 290 psi to bars); on the other hand, the source of compressed air 19 is so dimensioned that compressed air (or another gas of suitable composition) is forced through the spray-gun union 18 under a pressure of the orderof 22 to 29 psi (1.5 to 2 bars).

Under these conditions, when the user of this spraygun depresses the control lever of trigger 1 1, the movement of this member about its fulcrum 11a opens the compressed-air valve 33 and unseats the needle valve or like valve member 9 carried by the end of sliding rod 10. Thus, paint under pressure is fed via union 5 to chamber 4, and from this chamber through

bores

8a, 8b and 12a to the nozzle 13 from which it is sprayed through the atomizing slot 13a in the form of a flat, fanshaped jet of liquid paint extending in a plane perpendicular to the plane of FIG. 1. This flat, fan-shaped liquid jet of paint is atomized or pulverized immediately into very fine particles by the double action of the tapered jet of compressed air issuing from the annular passage 23, this air jet being guided by the frustoconical lateral surface of said nozzle 13 so as to impinge on the liquid jet of paint slightly ahead of the outlet of the atomizing slot 13a, on the one hand, and by the two jets emerging at right angles to the flat jet of paint from the pair of

passages

28a and 28b, which are subsequently deflected by the frustoconical surface 21a of the air head 21 so that they converge towards a point also located slightly ahead of the atomizing slot 13a, on the other hand.

In FIGS 4 and 5, the component elements homologucs to those illustrated in FIG. 1 are designated by the same reference numerals as in this last-mentioned figure.

The form of embodiment of the atomizing head illustrated in FIG. 4 differs from that illustrated in FIG. 2 only by the following features: the annular chamber is supplied with compressed air via a passage 31 formed in the lower portion of the spray-gun body 2 and receiving compressed air in parallel with the passage 17. On the other hand, the front face of air head 21 is not provided with a frustoconical surface (for instance like the surface 21a of FIG. 2), both

passages

28a and 28b being somewhat inclined to the perpendicular to the flat jet of paint issuing from the atomizing slot 13a in order to produce air jets converging towards said flat jet of paint at a point located well ahead of said atomizing slot 13a.

The embodiment of FIG. 5 differs from that of FIG. 4 only by the provision, the front portion of the air head 21, of a pair of

additional passages

32a and 32b substantially parallel to the

passage

28a and 28b of the preceding embodiments and supplied with compressed air from the same

longitudinal passages

29a and 29b, these additional passages 32a and 3219 being so disposed as to produce air jets converging towards the flat, already partially atomized jet of paint, at a point relative remote from the outlet of said atomizing slot 130.

In the foregoing it is disclosed that advantageous results can be derived from the deflection of a pair of additional airjets by means of a frustoconical surface before causing said jets to impinge on the flat faces of the jet of paint.

In a first form of embodiment (FIGS. 2 and 6) the additional airjets F are deflected by a frustoconical por tion 21a of air head 21 covering the atomizing head (comprising the nozzle 13 surrounded by an annular member 14 of which the frustoconical front face constitutes an extension of the lateral frustoconical surface of said nozzle 13) and converging towards the nozzle axis.

The atomizing head and the air head are so designed that two additional air jets F before impinging on the paint jet are deflected by the frustoconical surface of nozzle 13 and the annular member 14 surrounding this nozzle.

To this end, the free end 14a of annular member 14 surrounding the nozzle 13 projects somewhat to the exterior of the central aperture 21b of said air head and the pair of

passages

28a, 28b are so directed as to converge towards the frustoconical surface of nozzle 13 and said annular member 14, between said central aperture 21b of said air head and the nozzle atomizing slot In the modified embodiment illustrated in FIG. 7 the

passages

28a and 28b of the air head are directed at right angles to the axis of said atomizing head.

Furthermore, in the modified structure illustrated in FIG. 8 the frustum of a cone formed by the nozzle 13 and annular member 14 has a more acute angle so that the passages 28a and 2817 can be set at an angle with respect to the axis of said atomizing head.

FIGS. 9 and 10 illustrate two possible forms of embodiments of an electrostatic version of this spray-gun.

In the example illustrated in FIG. 9 two electrodes E and E are embedded in the air head 21 consisting pref erably of electrically insulating material, i.e. on the one hand a primary electrode E comprising a portion embedded in the upper lobe of the air head 21 so as to contact the front end of the aforesaid key 27 and consisting of electrically conducting material, and a portion emerging from said upper lobe of air head 21 in a direction at right angles to the atomizing slot 13a and having a pointed end, and on the other hand a secondary electrode E projecting from the lower lobe of said air head 21 and registering with the pointed end of the primary electrode E secured to the embedded portion of this secondary electrode E is a conducting pin E of which the pointed end emerging from the front face of said air head 21, notably in a suitable cavity, is directed towards the front of the spray-gun (i.e. to the left as seen in FIG. 9) or towards the articles or pieces to be painted. This secondary electrode E is thus completely insulated from the other conducting components of the spray-gun.

According to this invention, the point of the primary electrode E and the blunted end of the secondary electrode E are aligned with each other at right angles to the horizontal plane containing the atomizing slot 13a, which is the plane in which said atomizing slot 13a projects a flat jet of paint, and the dimensions of the two electrodes E and E are such that their registering ends are positioned on either side of the flat jet of paint, at relatively short distances from the two flat faces of this jet but outside this jet proper. In a typical form of embodiment of the spray-gun according to this invention, as shown in FIG. 9, the two registering ends of electrodes E, and E lie in a common vertical plane spaced by a distance of approximatively mm (0.6 inch) from the atomizing slot 13a opening at the front end of nozzle 13, and said registering ends of the two electrodes are also spaced from each other by a distance of about 7 mm 0.275 inch).

The electrostatic atomizing apparatus according to this invention, of which the spray-gun proper is illustrated in FIG. 9, comprises in addition a medium voltage electric current generator 34 connected via a flexible insulated cable to the spray-gun, and in this form of embodiment the conductor 34a transmitting the medium voltage generated by said generator 34 to the spray-gun proper is disposed in a manner known per se coaxially within the flexible hose 18a supplying compressed air thereto. Inside the spray-gun body 2 the conductor 340 also disposed coaxially to the compressed-air passage 35b is connected to the electric inlet of a voltage booster 36 of the type known in the art under the name of electro-gas-dynamic generator; the electric outlet of this voltage booster 36 is adapted to transmit via a conductor 37 the high voltage generated by this booster 36 to the tubular or socket member 8 made of conducting material, like the ring 12 and annular member 14; consequently, the high voltage applied to said conductor 37 is transmitted via said

members

8 and 12 to the hydrostatic atomizing nozzle 13 and via the

other members

14 and 27 to the primary electrode E The voltage booster 36 of the electro-gas-dynamic type is supplied with compressed air via passage 35b, and at least one fraction of the compressed air issuing therefrom is directed to passage 17 via passage 35a, the remaining fraction being vented to the atmosphere, if desired.

The paint atomizing apparatus according to this invention, as illustrated diagrammatically in FIG. 9 and described hereinabove, is operated as follows: The paint pump 6 is selected and adjusted to deliver liquid paint through the spray-gun union 5 under a pressure of the order of 430 to 750 psi to 50 bars), and on the other hand the source of compressed air 19 is so dimensioned that it delivers compressed air or another suitable gasunder pressure to the union 18, the gas pressure being such that the compressed air emerging from the voltage booster 36 has a pressure of the order of 7 to 15 psi'(O.5 to 1 bar).

Under these conditions, when the spray-gun operator depresses the trigger or control lever l 1 the movement there of about its fulcrum 11a unseats on the one hand the needle or like valve 9 carried by the front end of sliding rod 10 and on the other hand the compressed air valve 20. Thus, the paint fed through union 5 to chamber 4 flows from this chamber through the passages 81) and 12a to nozzle 13 by which it is sprayed through the atomizing slot 13a in the form of a liquid, flat and fanshaped jet of paint formed in a plane perpendicular to that of FIG. 9. After travelling a distance of less than 0.5 inch through the atmosphere the jet of liquid paint is no more homogeneous and is broken into fragments.

This effect takes place or at least begins at the level of a well-defined line usually referred to as the atomization front. Now the conditions of operation of the spray-gun according to this invention, that is, notably, the feed pressures for the paint and compressed air, respectively are so adjusted that the atomization front builds up substantially in the vertical plane containing the tips of electrodes E and E or slightly upstream of i this plane (in which case the electrodes are located shortly upstream of the atomization front). The reasons for this specific c hoice have been disclosed in the foregoing. Notwithstanding the relatively low pressure, well I below 1,450 psi bars), of the paint sprayed by the nozzle 13 of the spray-gun according to the present invention, the flat jet of liquid paint atomized through the slot of said nozzle is divided into very fine particles due to the dual action of the tapered jet of compressed air emerging from the annular passage 23 and, being guided by the lateral frustoconical surface of said nozzle l3, impinges on the liquid jet of paint shortly ahead of the atomizing slot 13a, on the one hand, and of the pair of compressed-air jets emerging from

passages

28a and 28b and converging to a point also located shortly ahead of the atomizing slot 13a, on the other hand. Due to the high voltage to which the primary electrode E is brought in relation to the ground, and to the objects to be painted and receiving the same electric voltage, the tip of the primary electrode E emits by corona action electric discharges collected nearly completely by the blunted tip of the secondary electrode E after flowing through the thickness of the flat, fan-like jet paint sprayed through the atomizing nozzle 13, substantially at the level of said atomization front. These conditions are the most adequate for charging very efficiently the particles of paint, whether atomized or in the course of atomization, so as to expose them to the aforementioned electric directing field. The charges collected by the electrically insulated secondary electrode E bring the potential thereof to a relatively high value, slightly below that of the primary electrode E so that between the blunted tip of secondary electrode E and the ground, or the objects receiving the same potential, as the ground, notably the articles to be painted, an intense electric field is created whereby the atomized and ionized paint particles are directed towards said objects or articles at ground potential. The secondary electrode E receiving a continuous flux of electric charges will preserve nevertheless a substantially constant potential because its blunted tip E emits a corresponding continuous charge flux towards the nearest objects or articles at ground potential.

Of course, the present invention should not be construed as being strictly limited by the specific forms of embodiment illustrated and described herein, for it includes likewise all modifications and variations within its scope. Thus, the number and relative arrangement of passages such as 28a and 28b are optional; these passages, instead of being disposed only in the bissecting plane of the fan-shaped paint jet (which is the plane of FIG. 1 may be disposed also or exclusively in another plane, for example the plane of the flat jet of paint. Instead of having an annular outlet of uniform width, the annular passage 23 may comprise locally widened section at least near its outlet orifice, for example in the plane of the jet of paint, or in the plane perpendicular thereto (i.e. the plane of FIG. 1). With these arrangements it is thus possible notably to reinforce locally the action exerted by the frustoconical air jet produced through the annular passage 23 in such directions as required by this reinforcement, for example to prevent the development of horns, i.e. thicker beads along the edges of the flat jet of atomized paint.

Although the primary and secondary electrode arrangement illustrated in FIG. 9 is particularly advantageous, many modifications may be brought thereto without departing from the basic principles of the invention. Thus, the shape and relative arrangement of the primary and secondary electrodes E and E are also optional, and a plurality of primary electrodes, and possibly of secondary electrodes, may be contemplated. The only requirement to be met in this matter and within the scope of the invention is that the tips or ends of all the electrodes contemplated be each located close to one of the flat faces of the paint jet, externally of this jet and substantially level with its atomization front, or slightly downstream of this front. It is also within the scope of this invention to provide a spraygun of the type illustrated in FIG. 9 but free of primary and secondary electrodes, only the atomizing nozzle 13 of electrically conducting material being brought to a high direct current voltage.

The conditions of operation of the paint atomizing apparatus according to this invention should not be limited to those specifically described hereinabove with reference to the first form of embodiment shown in FIG. 1; thus, the paint feed pressure may as a rule be selected in the range of 145 to 1450 psi (10 to I bars), and the pressure of the compressed air supplied to passage 17 may range from 7 to 29 psi (0.5 to 2 bars).

Instead of consisting of an eleetro-gas-dynamic type generator, the voltage booster 36 incorporated in the spray-gun body 2 may be of any other suitable type operating in a purely electrical manner. Instead of incorporating such voltage booster in the spray-gun proper, the latter may also be supplied from a high-voltage current generator disposed externally of the gun, but in this case a thicker, less flexible, cumbersome and expensive cable characterised by a high degree of electric insulation must be used. In these last-mentioned two alternatives the supply of compressed air through the feed union of the spray-gun according to this invention (i.e. union 18, FIG. 1) must take place at a pressure of 7 to 29 psi (0.5 to 2 bars).

If the paint pressure is within the range of 430 to 750 psi (30 to 50 bars), with a compressed air supply within the range of 7 to 15 psi (0.5 to 1 bar), the operation of the spray-gun apparatus according to this invention differs considerably from the one described hereinabove in connection with the form of embodiment illustrated in FIG. 1. In fact, the considerably higher pressure of the paint sprayed by the hydrostatic atomizing nozzle will then assist in producing a very fine atomization of the liquid paint jet issuing from said nozzle.

If paint pressures of the order of 145 to 290 psi 10 to 20 bars) are used, the spray-gun apparatus according to this invention may include for instance a paint pump of the pneumatic motor type producing a pressure multiplication in the ratio of 3 or 4, and with higher paint pressures, of the order of 430 to 750 psi (30 to 50 bars) the apparatus according to the present invention may comprise a pump of same type providing a pressure multiplication ratio of the order of 10 to 15; the use of such pumps is particularly advantageous if compared with purely hydrostatic spraying apparatus, which require pumps capable of producing a pressure multiplication ratio of the order of 30.

Of course, many modifications may be brought to the specific forms of embodiment described hereinabove with reference to the attached drawings without necessarily departing from the scope of the present invention. Thus, notably, the nozzle 13 and annular member 14 could form a single member in case the use of carbide steel were not absolutely necessary for making said nozzle. Similarly, the modified embodiment illustrated in FIG. 10, wherein the atomizing head abutes with its shoulder 14b against a shoulder 21b of the air head, could be incorporated in the device illustrated in FIGS. l-9.

What is claimed is:

1. Apparatus for atomizing paint or a similar liquid, which comprises a spray-gun of which the atomizing head incorporates a nozzle for the hydrostatic atomization of paint projecting a flat, fan-shaped jet of paint, at least two orifices disposed on either side of said nozzle for directing air jets onto said paint jet, means for feeding paint to said nozzle under a pressure of 10 to bars and means for feeding compressed air to said orifices under a pressure of 0.5 to 2 bars, the atomizing head further comprisng an annular orifice concentric to said hydrostatic atomizing nozzle supplied with compressed air substantially under the same pressure as the other orifices aforesaid, and thus producing a hollow frustoconical air jet converging to a point adjacent the outlet orifice of said nozzle.

2. Apparatus according to claim 1 comprising means for bringing said nozzle, made of conducting material, to a direct voltage of relatively high value in relation to the ground and to the objects to be painted.

3. Apparatus acccording to claim 1 in which the means to supplying paint to the nozzle of said atomizing head supply it under a pressure of 10 to 20 bars and the means for feeding compressed air to said orifices supply it under a pressure of 1.5 to 2 bars.

4. Apparatus according to claim 1 in which the means to supply paint to the nozzle of said atomizing head feed it under a pressure of 30 to 50 bars and the means for feeding compressed air to said orifices feed it under a pressure of 0.5 to 1 bar.

5. Apparatus according to claim 1 in which the annular orifice comprises local widened portions.

6. Apparatus according to claim 1 in which the annular orifice is the outlet of an annular passage bounded by two frustoconical surfaces formed coaxially to said hydrostatic atomizing nozzle.

7. Apparatus according to claim 1, in which the annular orifice is the outlet of an annular passage and the surface defining internally said annular passage comprises at least one fraction of the frustoconical lateral surface of the hydrostatic atomizing nozzle which merges continuously into the front, part-spherical surface of said nozzle which comprises its outlet orifice, the surface defining externally said annular passage consisting of an internal, frustoconical surface of one element of said atomizing head in which the other orifices supplied with compressed air are formed.

8. Apparatus according to claim 1, in which the two orifices provided for directing airjets onto said paintjet are disposed symmetrically in relation to the fanshaped jet of paint, in the bissecting plane thereof, so that their air jets converge towards a first point of the axis of said hydrostatic atomizing nozzle which is coincident with, or adjacent to the outlet orifice of said noz- 210.

9. Apparatus according to claim 1, in which the two orifices provided for directing airjets onto said paintjet are disposed symmetrically in relation to the fanshaped jet of paint, in'the bissecting plane thereof, so that their air jets converge towards a first point of the axis of said hydrostatic atomizing nozzle which is coincident with, or adjacent to the outlet orifice of said nozzle, and said atomizing head comprises about said annular orifice concentric to said atomizing nozzle a frustoconical surface so disposed as to deflect the air jets issuing from the other two orifices towards the aforesaid first point of the axis of said hydrostatic atomizing nozzle.

10. Apparatus according to claim 1, in which the two orifices provided for directing air jets onto said paint jet are disposed symmetrically in relation to the fanshaped jet of paint, in the bissecting plane thereof, so that their air jets converge towards a first point of the axis of said hydrostatic atomizing nozzle which is coincident with, or adjacent to the outlet orifice of said nozzle, and said atomizing head comprises at least two other orifices so disposed as to direct convergent air jets towards a second point on the axis of said hydrostatic atomizing nozzle which is located downstream in relation to said first point.

11. Apparatus according to claim 1 in which the two orifices provided for directing air jets onto said paint jet are disposed symmetrically in relation to the fanshaped jet of paint, in the bissecting plane thereof, so that their air jets converge towards a first point of the axis of said hydrostatic atomizing nozzle which is coincident with, or adjacent to the outlet orifice of said nozzle, and the frustoconical surface of nozzle and the annular member surrounding this nozzle deflect the two additional air jets before they impringe on the paint jet.

12. Apparatus according to claim 1, in which the two orifices provided for directing air jets onto said paintjet are disposed symmetrically in relation to the fanshaped jet of paint, in the bissecting plane thereof, so that their air jets converge towards a first point of the axis of said hydrostatic atomizing nozzle which is coincident with, or adjacent to the outlet orifice of said nozzle, and the frustoconical surface of nozzle and the annular member surrounding this nozzle deflect the two additional air jets before they impringe on the paint jet, and the free end of said annular member surrounding said nozzle projects externally of the central aperture of said air head and the two passages conveying said additional air jets are directed to converge towards said frustoconical nozzle surface and the annular member surrounding said nozzle, between said central aperture of the air head and the atomizing slot of said nozzle.

13. Apparatus according to claim 1 in which the atomized head further comprises at least one primary, needle-shaped electrode brought to the same electric voltage at the hydrostatic atomizing nozzle and disposed outside said flat jet of paint so that its tip be positioned close to one of the flat faces of said paint jet, at the level of the paint atomization front, or slightly downstream thereof, together with at least one insulated secondary electrode comprising a first blunted end registering with the tip of said primary electrode, on the opposite side of said flat paint jet, outside this jet but at a short distance therefrom, and a second pointed end relatively spaced from said paint jet so as to be directed preferentially ahead of the spray-gun towards the objects to be painted.

14. Apparatus according to claim 1 in which the at omized head further comprises at least one primary, needle-shaped electrode brought to the same electric voltage as the hydrostatic atomizing nozzle and disposed outside said flat jet of paint so that its tip be positioned close to one of the flat faces of said paint jet, at the level of the paint atomization front, or slightly downstream thereof, together with at least one insulated secondary electrode comprising a first blunted end registering with the tip of said primary electrode, on the opposite side of said flat paint jet, outside this jet but at a short distance therefrom, and a second pointed end relatively spaced from said paint jet so as to be directed preferentially ahead of the spray-gun towards the objects to be painted and in which the tip of said primary electrode and the first, blunted end of said secondary electrode are aligned at right angles to said flat jet of paint.

15. A spray-gun for atomizng paint or another similar liquid, which comprises a nozzle for the hydrostatic atomization adapted to project a flat, fan-like jet of liquid, and at least two orifices disposed on either side of said nozzle and symmetrically to said flat liquid jet in order to direct air jets in a plane at right angles to that of said flat liquid jet, wherein said nozzle comprises a front surface symmetrical in relation to the plane of said flat liquid jet and inclined towards the front and from the outside towards said flat liquid jet; said lateral orifices being so arranged as to project their air jets against said inclined nozzle surface whereby the said inclined noule surface will deflect said air jets into two sheets surrounding said flat liquid jet.

16. A spray-gun as set forth in claim 15, wherein the front surface of said nozzle against which said lateral orifice project their air jet is conical.

Claims

1. Apparatus for atomizing paint or a similar liquid, which comprises a spray-gun of which the atomizing head incorporates a nozzle for the hydrostatic atomization of paint projecting a flat, fan-shaped jet of paint, at least two orifices disposed on either side of said nozzle for directing air jets onto said paint jet, means for feeding paint to said nozzle under a pressure of 10 to 100 bars and means for feeding compressed air to said orifices under a pressure of 0.5 to 2 bars, the atomizing head further comprisng an annular orifice concentric to said hydrostatic atomizing nozzle supplied with compressed air substantially under the same pressurE as the other orifices aforesaid, and thus producing a hollow frustoconical air jet converging to a point adjacent the outlet orifice of said nozzle.

8. Apparatus according to claim 1, in which the two orifices provided for directing airjets onto said paint jet are disposed symmetrically in relation to the fan-shaped jet of paint, in the bissecting plane thereof, so that their air jets converge towards a first point of the axis of said hydrostatic atomizing nozzle which is coincident with, or adjacent to the outlet orifice of said nozzle.

9. Apparatus according to claim 1, in which the two orifices provided for directing air jets onto said paint jet are disposed symmetrically in relation to the fan-shaped jet of paint, in the bissecting plane thereof, so that their air jets converge towards a first point of the axis of said hydrostatic atomizing nozzle which is coincident with, or adjacent to the outlet orifice of said nozzle, and said atomizing head comprises about said annular orifice concentric to said atomizing nozzle a frustoconical surface so disposed as to deflect the air jets issuing from the other two orifices towards the aforesaid first point of the axis of said hydrostatic atomizing nozzle.

10. Apparatus according to claim 1, in which the two orifices provided for directing air jets onto said paint jet are disposed symmetrically in relation to the fan-shaped jet of paint, in the bissecting plane thereof, so that their air jets converge towards a first point of the axis of said hydrostatic atomizing nozzle which is coincident with, or adjacent to the outlet orifice of said nozzle, and said atomizing head comprises at least two other orifices so disposed as to direct convergent air jets towards a second point on the axis of said hydrostatic atomizing nozzle which is located downstream in relation to said first point.

11. Apparatus according to claim 1 in which the two orifices provided for directing air jets onto said paint jet are disposed symmetrically in relation to the fan-shaped jet of paint, in the bissecting plane thereof, so that their air jets converge towards a first point of the axis of said hydrostatic atomizing nozzle which is coincident with, or adjacent to the outlet orifice of said nozzle, and the frustoconical surface of nozzle and the annular member surrounding this nozzle deflect the two additional air jets bEfore they impringe on the paint jet.

12. Apparatus according to claim 1, in which the two orifices provided for directing air jets onto said paint jet are disposed symmetrically in relation to the fan-shaped jet of paint, in the bissecting plane thereof, so that their air jets converge towards a first point of the axis of said hydrostatic atomizing nozzle which is coincident with, or adjacent to the outlet orifice of said nozzle, and the frustoconical surface of nozzle and the annular member surrounding this nozzle deflect the two additional air jets before they impringe on the paint jet, and the free end of said annular member surrounding said nozzle projects externally of the central aperture of said air head and the two passages conveying said additional air jets are directed to converge towards said frustoconical nozzle surface and the annular member surrounding said nozzle, between said central aperture of the air head and the atomizing slot of said nozzle.

14. Apparatus according to claim 1 in which the atomized head further comprises at least one primary, needle-shaped electrode brought to the same electric voltage as the hydrostatic atomizing nozzle and disposed outside said flat jet of paint so that its tip be positioned close to one of the flat faces of said paint jet, at the level of the paint atomization front, or slightly downstream thereof, together with at least one insulated secondary electrode comprising a first blunted end registering with the tip of said primary electrode, on the opposite side of said flat paint jet, outside this jet but at a short distance therefrom, and a second pointed end relatively spaced from said paint jet so as to be directed preferentially ahead of the spray-gun towards the objects to be painted and in which the tip of said primary electrode and the first, blunted end of said secondary electrode are aligned at right angles to said flat jet of paint.

15. A spray-gun for atomizng paint or another similar liquid, which comprises a nozzle for the hydrostatic atomization adapted to project a flat, fan-like jet of liquid, and at least two orifices disposed on either side of said nozzle and symmetrically to said flat liquid jet in order to direct air jets in a plane at right angles to that of said flat liquid jet, wherein said nozzle comprises a front surface symmetrical in relation to the plane of said flat liquid jet and inclined towards the front and from the outside towards said flat liquid jet; said lateral orifices being so arranged as to project their air jets against said inclined nozzle surface whereby the said inclined nozzle surface will deflect said air jets into two sheets surrounding said flat liquid jet.