EP0281438B1 - Device for the electrostatic spraying of powdery products - Google Patents

Description

The invention relates to a device for electrostatic projection of powdered product and more particularly a coating product conveyed by air, the invention relates more particularly to an improvement making it possible to avoid that a counter-electrode (including the role is, in particular, to capture the free ions of the air-powder mixture ejected towards an object to be covered), does not gradually lose its effectiveness by being covered by the coating product forming an insulating film.

Electrostatic powdering is commonly used in industry, in particular to cover objects with a hot-melt paint applied in powder. When such objects are completely covered with a sufficient layer of powder, they are transported to an oven where the powder coating becomes a homogeneous and resistant layer of paint, by melting the grains of powder together followed in general by a polymerization.

To electrically charge the powder particles of the air-powder mixture, an electric charge field can be created between a charging electrode placed in the air-powder mixture and a counter electrode separated from the charging electrode by a distance of a few centimeters. It is also necessary to create a deposit field between the projection device and the object to be covered. This is generally achieved by bringing the charging electrode to a high electrical voltage and connecting the object to earth. The counter-electrode can itself be connected to earth or brought to an intermediate potential, to create the desired field, taking into account the dimensions of the projector and in particular the distance between the two electrodes.

From this point of view, it is sought to develop devices implementing voltages which are not excessively high. It is particularly desirable to create a charge field over a distance of 30 to 40 mm with a potential difference of around 30 kV between the electrodes.

It was found that, if free ions propagated at the same time as the air-powder mixture towards the object to be covered, by being entrained by the deposition field, differences in concentration of powder deposition could occur between the reliefs and the object's fractures. These differences in powder concentration are obviously the source of differences in thickness in the paint layer, after passing through the oven. It is therefore desirable that substantially all of the free ions which could not charge the powder are recovered by an ion trap, in this case the counter-electrode. Several counter-electrode configurations have been proposed to best ensure the "recovery" of free ions. Many of the solutions proposed place the counter-electrode inside the air-powder mixture circulation conduit or in direct communication with the interior of this conduit. These solutions are generally unsatisfactory due to the fact that the powder comes more or less quickly to cover the counter-electrode, leading to the formation of an insulating coating preventing it from playing its role of ion trap. It is assumed that one of the phenomena in question is a local fusion of the powder grains resulting from their impact energy on the counter-electrode. To avoid such agglomeration of powder on the counter-electrode, it has been proposed to clean it permanently by a circulation of clean air and even to provide a porous counter-electrode permanently traversed by air. A solution of this type is for example described in French patent No. 2 283 729. It has also been proposed to place the electrode outside the air-powder mixture circulation duct, but it has not been possible, until 'Now prevent particles of powder entrained by vortices from covering the counter electrode. In this connection, the American patent N ° 4 228 961 proposes a structure with an axial cylindrical counter-electrode, cleaned by an air jet maintained in a very small annular space around the counter-electrode. The permanent cleaning of the counter electrode by this air jet has not given good results and this failure can be attributed to the fact that the air jet can itself promote the formation of vortices in the vicinity of the counter -electrode. In addition, it seems that the shape of this counter-electrode and the small cross-section of the passage which the free ions must take to come and meet the counter-electrode, considerably reduce the effectiveness of the latter as an ion trap. In addition, a passage of such a small section can be closed in use.

The invention provides a new arrangement of an electrostatic powder projection device, comprising a counter-electrode situated outside the circulation duct, of a shape and dimensions suitable for enabling it to effectively play its role of ion trap, said counter -electrode being associated with blowing means ensuring, among other things, its permanent cleaning. The invention relates in particular to an arrangement making it possible to prevent vortices of air-powder mixture from forming in the immediate vicinity of the counter electrode, in particular due to the very flow of the air responsible for cleaning this counter electrode .

In this spirit, the invention therefore relates to a device for electrostatic projection of powdered product, in particular a coating product, of the type comprising a duct for circulating an air-powder mixture, provided with an ejection orifice at a end, a charging electrode disposed in the vicinity of said ejection orifice and a counter electrode arranged coaxially with said charging electrode in an open elongated annular space, coaxial with said circulation conduit and axially withdrawn from the orifice ejection of the latter, this annular space being connected to air blowing means and a potential difference being applied between the two electrodes to create an electric field capable of causing free ions towards said counter-electrode, characterized in that said counter-electrode is situated at the bottom of said annular space with respect to its opening and extends substantially perpendicular to the axis of the latter and in that said air blowing means open into said annular space in the vicinity of said counter-electrode.

In a simple manner, the abovementioned annular space is defined between the external surface of said conduit for circulation of the air-powder mixture and an outer sleeve and coaxial with this conduit.

Thus, the counter-electrode can, without drawback, have a relatively large annular surface (perpendicular to the axis of the device) and arranged so that the free ions remaining in the space where the charge field prevails, are almost - all attracted to it. Because the air is channeled into said relatively long annular space, the powder particles which could be attracted to the counter-electrode due to their charge, are repelled by the force of the air jet. The latter's role is therefore no longer only to clean the counter-electrode, but also to prevent grains of powder from reaching it. In addition, eddies capable of transporting powder to the counter-electrode are no longer to be feared since no call for polluted ambient air is possible in the vicinity thereof.

The invention also relates to a particular embodiment of such an arrangement which provides a surprising improvement in the performance of the device.

A first means of this variant consists of a tapered configuration, in particular frustoconical, of the sleeve for confining the annular space formed in front of the counter-electrode: this results in an advantageous advantageous reduction in air consumption and better penetration of the coating on parts with cavities.

According to another arrangement of this variant, the abovementioned confining sleeve is pierced with a plurality of orifices opening out near the counter-electrode: these orifices create exhaust paths for the counter-electrode scanning air according to a plurality of directions transverse to the direction of projection. These scavenging air escape channels constitute a means of improving the efficiency of the counter-electrode in its ion trap action with a view to the selective capture of free ions in the air-powder mixture ejected towards the object to be covered.

According to another arrangement of the variant, the terminal part of the air-powder mixing duct is given a very tapered shape with the advantage of avoiding practically any risk of accumulation of powder at the ejection orifice which is a source of improvement in the quality of the coating obtained by reducing the risk of projection of agglomerates.

• _ la figure 1 est une vue schématique en coupe longitudinale d'un mode de réalisation possible d'un dispositif de projection électrostatique de poudre conforme à l'invention;
• _ la figure 2 est une vue en plan d'un autre dispositif de projection conforme à l'invention; et
• _ la figure 3 est une vue en coupe longitudinale selon le plan III-III de la figure 2.

The invention will be better understood and other advantages thereof will appear more clearly in the light of the following description of an embodiment in accordance with its principle, given solely by way of example and made with reference to annexed drawings in which:

• _ Figure 1 is a schematic view in longitudinal section of a possible embodiment of an electrostatic powder spraying device according to the invention;
• _ Figure 2 is a plan view of another projection device according to the invention; and
• _ Figure 3 is a longitudinal sectional view along the plane III-III of Figure 2.

The powder spraying device 11, as shown in FIG. 1, mainly consists of the assembly of a metallic body 12 and connected to the earth in the example described containing a certain number of passages and cavities and a pipe 13 for circulating the tubular and cylindrical air-powder mixture, connected sealingly to the body 12 by one of its ends, with the interposition of an annular seal 14. The insulating conduit 13 has at its other end an orifice 15 for ejecting said air-powder mixture. The body 12 and the circulation duct 13 are crossed by an insulating rod 16, arranged coaxially with the duct 13, with a tubular structure housing a high-voltage cable 17 whose core 17 a is in contact with a spring 18 establishing the electrical connection with one end of a resistor 19. The other end of this resistor is in contact with a charging electrode 20 comprising a disc-shaped collar 21 coaxial with the rod 16 and the conduit 13 and extending substantially perpendicular to this axis. Its circular outline emerges from the lateral surface of the insulating rod 16. The latter is held in its axial position by being engaged in a bore 22 of the body 12 on the one hand, and in the central bore of a part 24 provided with 'radial fins 26 bearing against the internal wall of the conduit 13, on the other hand. The charging electrode 20 is located outside the ejection orifice 15, in the axial extension of the circulation duct 13 of the air-powder mixture. In a manner known per se, the insulating rod is extended in the direction of ejection of the air-powder mixture by an enlarged part 27 forming a deflector. The charging electrode 20 is here interposed between the cylindrical rod 16 and the enlarged part 27 and constitutes, by virtue of threaded portions located in the extension of one another, the means of assembly of these two elements. The body 12 is provided with a connector 29 by which the air-powder mixture is introduced into an inclined passage 30. The latter communicates with the end of the circulation duct 13, assembled to said body 12.

Another connector (not visible in the drawing) communicates with holes 31, 32 of the body 12. It is intended to be connected to a source of compressed air. The bore 32 opens into an annular housing 34 defined between the external surface of the conduit 13 and a conduit 35, generally cylindrical, attached in a sealed manner to the body 12, by one of its ends, with the interposition of a seal 36. The arrangement (31, 32, 34, 35) forms blowing means for cleaning a counter-electrode 38 placed outside the circulation duct 13 so as to close one end of the annular housing 34. In the device described, the counter-electrode is porous, for example made of porous bronze. This feature is known per se. Thus, pressurized air injected into the annular housing 34 escapes through said counter electrode 38. The latter forms a flat annular wall bearing, internally, against a shoulder of the external surface of said circulation conduit 13 and, externally, against one end of the conduit element 35. The conduit 13 and the conduit element 35 are made of insulating material and the counter-electrode is brought to a selected potential, in this case the earth potential , by a spring 39 slightly compressed in the annular housing and coming into contact with the body 12.

According to an important characteristic of the invention, the counter-electrode, external to the conduit 13 and which extends perpendicular to the axis of the latter is protected by an external sleeve 40 which defines with the circulation conduit 13 and around that here, an elongated annular space 41. The latter is therefore located in the extension of the counter electrode in the direction of the charging electrode 20 and, because here the counter electrode is porous, the blowing means defined below above open into this annular space 41. The counter-electrode 38, in the form of a flat annular wall, therefore extends between the external surface of the circulation duct and one end of the external sleeve 40. To facilitate assembly, the element of conduit 35 and the sleeve 40 are located coaxially in the extension of one another and the counter-electrode 38 is clamped externally between the opposite ends of said conduit element 35 and said sleeve 40. To do this, a mounting ring 45, internally threaded, is screwed on an external threaded portion of the duct element 35 and it has an internal shoulder 46 at one end, which bears against an external shoulder 47 of the sleeve 40. Advantageously, the ring 40 and the sleeve 45 can be produced in one room.

It has been found that with this arrangement, the force imparted by the air flowing in the sleeve 40 to the charged particles (this force essentially depends on the speed of the air flow in the sleeve and on the surface of the particles) was greater than the force of electrical origin attracting these same charged particles. These conditions are only realized in the presence of the sleeve due to the channeling of the air escaping from the counter-electrode. Furthermore, no vortex (liable to entrain powder) can form in the vicinity of the counter-electrode, the vortex zones being, at worst, carried forward, in the vicinity of the end of the sleeve 40.

A metal rod 48, in electrical contact with the charging electrode 20 (and which can advantageously be an extension of the latter) emerges axially from the end of said enlarged part 27. This feature improves the configuration of the deposition field.

Of course, the invention is not limited to the embodiment which has just been described. The sleeve 40 is not necessarily cylindrical, it can be conical, widening in the direction of the counter electrode 38, which makes it possible to use a larger counter electrode. If the charging electrode is not brought to a very high high voltage, the counter electrode 38 can be earthed. On the other hand, if one wants to increase the voltage applied to the charging electrode, so as to increase the deposition field, one can avoid modifying the dimensions of the device, that is to say in particular the distance between the electrode load and the counter-electrode, by bringing said counter-electrode to an intermediate voltage. As an example, we try to maintain a difference of potential of 30 kV between the two electrodes. If it is desired to increase the deposition field, the charging electrode can be brought up to 90 kV, the counter-electrode then being brought to 60 kV. Other modifications are possible. In particular, the charging electrode 20 is not necessarily located at the junction between the cylindrical part of the rod 16 and the enlarged part 27. The charging electrode can in particular emerge at any point from said enlarged part and even be constituted by a simple metal disc covering the end of the deflector, the rod 48 then being removed. It is also possible to envisage making the charging electrode in the form of several points, for example three, regularly distributed angularly and projecting preferably at the periphery of the deflector. Finally, the counter electrode can be formed from another porous metal (for example porous stainless steel) or from a conductive synthetic material. Nor does it have to be porous; one could perfectly conceive a variant in which air passages would bypass it to establish air circulation between the housing 34 and the annular space 41.

In the embodiment of FIGS. 2 and 3, most of the elements of the device described with reference to FIG. 1 are recognized, the similar elements being designated by the same references increased by 100.

So to find the powder projection device 111, there is an assembly of a rear body 112 with a circulation duct 113 of air-powder mixture leading to an ejection orifice 115. There is an insulating rod 116 passing through the body 112 and the conduit 113 with the high-voltage cable 117, the core of which is placed by the spring 118 in electrical connection with a resistor 119, itself in contact with the charging electrode 120 comprising the flange 121, in rear of the flared end portion 127, forming a deflector.

It should be noted that the overshoot in direction radial of the flange 121 relative to the peripheral surface of the insulating rod 116 and its extension 127, is limited to the value just sufficient for the formation of aromas. This excess is of the order of a tenth of a millimeter.

The rear body 112 equipped with a support foot 110 is made in this case of insulating material. It is provided with the connector 129 by which the air-powder mixture is introduced into the inclined and flared passage 130 opening into the circulation duct 113. Another connection 131 connected to a source of compressed air gives access to a bore 132 which opens out. in the annular housing 134 defined between the external surface of the circulation conduit 113 and the external conduit element 135. There is at 138 the counter-electrode, for example made of porous bronze, which closes the anterior end of the annular passage 134. This counter-electrode is kept at ground potential by means of the spring 139 which here finds support on the rear body 112, by means of a conductive stud 150, itself connected to a connection terminal. the earth 151, by means of a spring 152 extending in a pipe 153 formed for this purpose in the body 112.

The counter-electrode 138 is held in abutment against bearing surfaces provided for this purpose on the circulation duct 113, on the one hand, and at the end of the outer duct 134, on the other hand, by means of a sleeve or skirt 140 Coming in one piece with a skirt nut 145. According to an important characteristic of this embodiment, the skirt sleeve 140 has a tapered frustoconical shape achieving a significant reduction in the section of the annular space 141. The frustoconical wall of the sleeve-skirt 140 is pierced with a circular row of holes 142 uniformly distributed over the periphery, opposite the counter-electrode 138.

The circulation duct 113 has, for its part, a very tapered end portion 113A with a wall external of frustoconical shape, the generator of this truncated cone forming with the axial direction an acute angle of small value, for example of about 5 °.

The frustoconical skirt sleeve 140 comprises, for its part, a tapered end portion delimited on the radially inner side by a frustoconical wall 143 substantially parallel to the frustoconical end surface 113A, so as to establish with the latter an annular passage of reduced cross section constituting a kind of exhaust nozzle T for the annular space 141.

• la voie axiale pour laquelle la section de passage est fortement réduite et la vitesse d'écoulement accélérée dans la tuyère d'échappement T;
• la voie radiale constituée par la série de trous 142 dont la présence a pour effet de réduire la pression susceptible de s'établir devant la contre-électrode en offrant une multiplicité de voies d'échappement à des vitesses d'écoulement qui seront naturellement fonction du diamètre de perçage des trous.

The annular space 141 confined by the sleeve-skirt 140 in front of the counter-electrode 138 around the conduit 113 for circulation of the mixture therefore offers the blowing air having passed through the counter-electrode the possibility of escaping by two different routes. :

• the axial channel for which the passage section is greatly reduced and the flow speed accelerated in the exhaust nozzle T;
• the radial channel formed by the series of holes 142 whose presence has the effect of reducing the pressure likely to be established in front of the counter-electrode by offering a multiplicity of exhaust channels at flow rates which will naturally depend on the hole drilling diameter.

Experience shows that this arrangement allows a considerable improvement in the performance of the projection device, for a number of reasons, the effects of which combine with one another in a particularly timely and advantageous manner.

There is, firstly, an extremely significant improvement in the quality of the coating obtained, in particular on parts having cavities thanks to a reduction in the risk of re-entrainment of the particles reached on the part by the blowing air coming from the counter -electrode.

All other things being equal, a significant part of this blowing air escapes laterally by the radial path offered by the holes 142, which decreases the quantity of air escaping in the direction of the part by the axial nozzle T.

Secondly, there is generally a reduction in air consumption.

Third, there is a surprising improvement in the efficiency of the counter electrode as a trap for free ions, with a reduction in the probability of unwanted capture of charged particles.

This efficient and regular collection of free ions results in particular in the establishment of a large and stable current, for example of 30 microamps between electrode and counter-electrode, which seems to be mainly attributable to beams of force lines and flow likely to pass through holes 142.

The protection against unwanted capture of charged particles is due to the accelerated flow velocities through the nozzle T in the axial direction where the electrostatic driving forces are highest. Protection is also ensured with regard to the risks of particles rising through the holes and the air exhaust speeds are lower, but so are the electrostatic driving forces.

The drilling diameter of the holes 142 will generally be between 1 and 4 mm. Excellent results have been obtained with a prototype provided with twelve holes of 3 mm in diameter, thus offering a total passage surface of approximately 85 mm 2, while the passage section offered by the nozzle T with an average diameter of 22 mm and a thickness of 2.5 mm was roughly equivalent.

The very tapered shape of the end portion 113 of the air-powder mixture circulation duct very effectively avoids the risks of powder accumulation and the projection of agglomerates. The tapered shape of the end portion 143 of the skirt sleeve 140 and the continuity of the latter with the skirt nut 145.

Claims (21)

1. An electrostatic spraying device for spraying product in powder form, in particular a coating product, of the type comprising a conduit (13, 113) for the flow of an air-powder mixture, provided with an ejection orifice (15, 115) at one end, a charging electrode (20, 120) disposed in the vicinity of said ejection orifice and a counter-electrode (38, 138) arranged coaxially with said charging electrode in an open elongated annular space, in coaxial relationship with said flow conduit and set back axially with respect to the ejection orifice of the latter, said annular space being connected to air blowing means (34, 134) and a potential difference being applied between the two electrodes to generate an electrical field capable of entraining free ions towards said counter-electrode, characterised in that said counter-electrode (38, 138) is disposed at the bottom of said annular space (41, 141) with respect to its opening and extends substantially perpendicularly to the axis of the annular space, and that said air blowing means open into said annular space in the vicinity of said counter-electrode.

2. A device according to claim 1 characterised in that said charging electrode (20) is disposed on the outside of said ejection orifice (15) in axial alignment with said flow conduit (13).

3. A device according to claim 1 or claim 2 characterised in that said annular space (41) is defined between the external surface of said flow conduit (13) and a sleeve (40) which is external to said conduit.

4. A device according to claim 3 characterised in that said counter-electrode (38) forms a flat annular wall extending at least between the external surface of said flow conduit and said external sleeve.

5. A device according to one of claims 1 to 4 characterized in that said counter-electrode (38) is a porous wall and that said blowing means comprise a conduit element (35) which is closed at one end by said porous wall, said conduit element being adapted to be connected to air supply means.

6. A device according to claim 5 characterised in that said conduit element (35) surrounds said flow conduit (13).

7. A device according to claim 5 or claim 6 characterised in that said conduit element (35) and said external sleeve (40) are disposed coaxially in alignment with each other and that said counter-electrode (38) is clamped externally between the facing ends of said conduit element and said external sleeve.

8. A device according to claim 7 characterised in that an internally screwthreaded mounting ring (45) is screwed on an external screwthreaded portion of said conduit element (35) and comprises an internal shoulder (46) at one end, said shoulder bearing against a corresponding external shoulder (4 7) of said external sleeve (40).

9. A device according to one of claims 1 to 8 characterised in that said charging electrode is provided with a portion (21) having a circular contour which is coaxial with said flow conduit.

10. A device according to claim 9 characterised in that said charging electrode comprises a metal disc (21), the circular edge of which emerges from the lateral surface of an insulating rod (16) disposed coaxially with said flow conduit.

11. A device according to claim 10 characterised in that in per se known manner said insulating rod (16) terminates in an enlarged portion (27) forming a deflector.

12. A device according to claim 11 characterised in that a metal rod (48) in electrical contact with said charging electrode (20) emerges axially from the end of said enlarged portion (27).

13. A device according to claim 10 characterised in that said charging electrode comprises a metal disc mounted at an enlarged end forming a deflector of an insulating rod disposed coaxially with said flow conduit.

14. A device according to one of claims 1 to 8 characterised in that said charging electrode comprises a plurality of regularly angularly distributed points.

15. A device according to claim 1 characterised by confinement of said annular space (141) by means of a sleeve skirt (140) of tapered shape defining a considerably reduced flow passage around the flow conduit while a circular array of holes (142) is provided in said sleeve skirt in the proximity of said counter-electrode.

16. A device according to claim 15 characterised by the provision at the end of the flow pipe (113) of a terminal portion (113A) which is highly tapered with a frustoconical radially external wall having a generatrix forming an acute angle to the axial direction.

17. A device according to claim 16 characterised in that the acute angle is about 5°.

18. A device according to claim 16 or claim 17 characterised by the provision at the end of the sleeve skirt of a tapered terminal portion delimited at the radially inward side by a frustoconical surface (143) substantially parallel to the external surface of the terminal portion (113A) of the flow pipe, said two coaxial surfaces thus providing a kind of exhaust nozzle T.

19. A device according to claim 18 characterised in that the array of holes (142) on the one band and the nozzle (T) on the other hand present flow areas of overall similar values.

20. A device according to claim 15 characterised in that the holes (14 2) are from 1 to 4 mm in diameter.

21. A device according to claim 15 characterised in that the holes (142) are 3 mm in diameter.

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