The known technology of electrostatic powder painting has been widely developed both in terms of plants as a whole and in terms of individual apparatuses for the application and necessary electrostatic charging of the powder. There are currently known two preferential approaches as regards systems for electrostatic charging of polymeric powder for painting surfaces or other objects. <BR> <BR> <P>A first technology uses a charging effect referred to as triboelectrification, i. e. , charging induced by the friction of particles of powder against an appropriate material that acquires or yields electrons (elementary negative charges) to the particles themselves so that these can be charged positively if they yield their own electrons or negatively with the acquisition of electrons yielded by the material on which they are rubbed.

Albeit valid when applied for charging the powder electrostatically, this system has manifested certain drawbacks. A first drawback is due to the fact that the triboelectric method enables electrical charging of amounts of powder emitted by an appropriate and standardized emitter (the so-called"triboelectric gun") in the region of 300 grams per minute and hence requires a large number of apparatuses (guns) for applying the paint in order to be able to cover large surfaces at a fast rate.

The triboelectric chargeability of the powder may moreover be affected by the conditions of surface humidity of the objects or by atmospheric humidity. At times, it is therefore not possible to charge the powder sufficiently to force it to adhere to the surfaces to be painted on account, as is known, of electrostatic induction, with consequent slowing-down or halting of production.

A second known technology exploits a mechanism of generation of ions due to a physical effect known as"corona discharge". An apparatus that uses said property is usually formed by an electrode, in the form of a pin, with a very small radius of curvature, connected to a high- voltage generator. As is known, the operation of said apparatus is due to the fact that the electrical charges tend to accumulate on the points of smaller radius of a conductive body, thus causing, in said points, a high accumulation of electrical charge per unit surface. Since the electrical field generated by an electrical charge is correlated thereto by the surface density of charge at the point in which said charge is located, in the immediate vicinity of said point the electrical field reaches very high values.

In these cases, if the radius of curvature of said points is sufficiently small there is a typical effect of attraction and repulsion of free ions in air in the immediate vicinity of said points.

Said effect of repulsion or attraction depends upon the sign of electrical charge of the free ions and upon the intensity and direction of the electrical field. The attraction or repulsion due to the force of the electrical field accelerates said ions, forcing them to collide with other molecules of air, thus generating other ions on account of the so-called avalanche effect, which produces a typical dielectric discharge in the air, referred to as corona discharge, around the surface of smaller radius of said points.

This technology is used as ionization system for charging, by capture, the particles of powder paint, which are appropriately made to pass through or around said ionized air.

Figure 1 is a schematic illustration of an example of a painting system based upon corona discharge according to the current state of the art. The system comprises an emitter of powder in the form of a gun 1, substantially a tube, in which the powder is introduced at one end and comes out at the other. Set therein, coaxially with respect to the direction of emission of the powder, on the outflow end 8, is an electrode in the form of a pin 2, connected to a generator 3, of high voltage usually of negative sign, by means of the cable 4.

A fluidized tank 5 of known technology contains the polymeric powder paint. Said powder is sent to the gun 1 through a flexible tube 6 by means of a Venturi pump 7, which sucks it from said tank 5. The pump is supplied with compressed air so that inside the internal pipe of the gun as far as its emission outlet 8 there will be a more or less diluted mixture 9 of air, usually referred to as"delivery air", and powder. <BR> <BR> <P>Corona discharge, i. e. , the generation of unipolar ions, in the case in point negative ions, represented schematically by the dots 11, by the pin electrode 2, charges the individual particles of powder, represented by the spheres 10, by the phenomenon of capture of said ions 11. The ions 11 deposit on the surface of said particles 10, transferring thereto an electrical charge that is directly proportional to the number of ions captured.

Said particles 10, once charged, under the effect of the delivery air of the powder, which impresses on them a certain speed, and of the force of acceleration impressed on them by the electrical field generated by the voltage of the electrode, represented visually by the arrows 12 and set between the electrode 2 and the object to be painted 13, are directed towards said object 13 and adhere thereto as a result of electrostatic induction.

Also this second technology, albeit valid, has manifested certain drawbacks, one of which is due to the high voltage, in the region of 50-80 kV, which must be applied to the electrode, and to the consequent intense electrical field associated thereto. The intense electrical field contributes to creating the so-called Faraday cages on the material that is set to be painted. In practice, slots and grooves or other geometrical features of said material can form screens, referred to as electrostatic screens, in which it is not possible, on account of the usual repulsive effects, to penetrate with the powder, thus leaving said portions of surface not coated with paint and hence not properly finished. Other deleterious effects due to the voltage and to the strong electrical field regard the finishing surface which, once it has been polymerized by being baked in an oven, presents a marked orange-peal effect, or a dense effect of so-called pinholes that are not deemed acceptable from the quality standpoint.

Furthermore, this method enables charging of amounts of powder emitted by a single emitter (corona gun) in the region of 400 grams per minute. If the aim is to achieve a high painting rate for large surfaces, it is necessary to increase the number of said guns. For example, if a surface of 10 square metres is to be painted in one minute with a thickness of powder of 60 um, there is required an emission rate of at least 2.2 kg/min, because only 40% of the powder paint is deposited. Calculating the volume of the powder deposited according to the surface, i. e. , 10 m2, and to the thickness, i. e. , 60 um, and considering a specific weight of approximately 1. 4 kg/dm3, we find that the amount of powder that must effectively be deposited is equal to 0.840 kg. Since the efficiency is equal to approximately 40%, we find that the amount of powder emitted must be equal to the value given above. Hence we shall need at least five of said guns, since each one of these is able to emit only 400 g of powder.

If it is necessary to increase the number of the powder emitters in order to increase the surface painted per unit time, also the electrical field 12 in the medium (air) set between the object to be painted 13 and the emitters 1 increases given that the electrical field in a point in space close to the surface of the object to be painted assumes a value equal to the vector sum of the electrical fields generated by each of the five electrodes of the powder emitters. As has already been said, the increase in the electrical field generated by the electrodes leads to phenomena that spoil the surface finish, once the coating has polymerized.

There is also known a simple physical law that links the radius of the electrode in an inversely proportional way to the intensity of the electrical field generated thereby, given the same supply voltage, so that, if the radius of the electrode increases, we should have an electrical field of very much lower intensity. Said configuration would not prove technically practicable since, by increasing the radius of the electrode, there would not be, given the same voltage, any corona discharge in the air surrounding the electrode. The electrical field generated would prove in fact insufficient for accelerating electrons or free ions in the vicinity of the electrode itself and for giving rise to the desired effect of multiplication of the ions and consequent charging of the powder by capture. By increasing the radius of the electrode, it would then be necessary, in order to obtain an electrical field such as to generate the corona discharge, to increase also the voltage connected thereto to levels that are not acceptable or expedient.

A possible simple solution, at least theoretically, for reduction of the electrical field generated, would be that of setting between the emitter generating the field and the object to be painted a substance other than air, which, as is known, has a dielectric constant equal to unity. Given the same physical conditions in fact, the intensity of an electrical field E generated by a voltage V at a distance d from the electrode has the value E = Vlsd, where s is the dielectric constant of <BR> <BR> the medium set between. It is thus evident that using a dielectric medium with s > 1, i. e. , with a dielectric constant greater than that of air, the resultant electrical field in a point of space at a distance d from the electrode is much smaller than it would be if the dielectric medium were air. Said result could be obtained by mixing with the air for suction of the powder from the fluidified tank a gas having a dielectric constant higher than that of the suction air, which would entail, however, an increase in plant and production costs.

On the basis of these considerations, the primary task forming the subject of the present invention is to provide a device for powder painting, as well as a process using said device, which will enable the drawbacks referred to above to be overcome.

In the framework of this task, the main purpose of the present invention is to provide a device for powder painting that will produce a number of ions sufficient to charge large amounts of powder emitted in unit time, so as to be able to increase the productivity of a painting plant.

Another purpose of the present invention is to provide a device for powder painting that will be able to reduce the effect of the electrical field generated by the electrodes to a level such as not to produce, or at least to minimize, the undesirable surface effects referred to above.

A further purpose of the present invention is to provide a device for powder painting that will enable reduction of the supply voltage of the electrodes to prevent phenomena of breakdown of air, i. e. , breakdown of the dielectric strength thereof, with the formation of electrical discharges, which may cause outbreak of fire.

Not the least important purpose of the present invention is to provide a device for powder painting, as well as a process using said device, which will present high reliability, relative ease of construction, and competitive costs.

The above task and the above purposes are achieved through a device for powder painting comprising an electrode for the generation of electrostatic charges, supplied by a voltage generator. The device according to the invention is characterized in that it comprises a body made of dielectric material having a cavity, in which said electrode is housed. Said cavity is designed to confine therein electrical charges generated by said electrode, and the body made of dielectric material comprises an opening, which sets the inside of said cavity in communication with the external environment and enables controlled emission, from within the cavity outwards, of said electrical charges.

Preferably, the device according to the invention comprises a tube having an internal surface that defines a substantially cylindrical cavity of radius R. In this case, advantageously, the electrode can be formed by a filiform element of radius r, with r < R, positioned within said cavity so that it is substantially coaxial thereto.

In a preferred embodiment of the device according to the invention, said opening, which sets the inside of said cavity in communication with the external environment, is formed by a side slit, which develops in a longitudinal direction along the body made of dielectric material.

The device according to the invention finds advantageous application in apparatus for powder painting, said apparatus moreover comprising a voltage generator connected to said electrode and a device for emission of the powder having an emission outlet located in the proximity of the opening that sets the inside of said cavity in communication with the external environment. Preferably, said apparatus comprises two devices according to the invention, arranged in positions geometrically opposite to the emission outlet of said emission device.

The present invention moreover relates to a method for the generation of electrical charges, particularly, in a powder-painting process, which comprises: supplying an electrode with an appropriate voltage by a generator, said electrode being positioned within a cavity, containing air, defined by the internal surface of a body made of dielectric material; generating electrical charges in said cavity, said cavity being designed to confine therein said charges so that the air contained therein may be considered conductive and equipotential with the electrode as far as the internal surface of said body; and issuing towards the external environment part of said charges through an opening so as not to jeopardize the conditions of conductivity and equipotentiality of the air in the cavity within the internal surface of said body.

Preferably, in the method according to the invention, the body made of dielectric material comprises a tube having an internal surface that defines a substantially cylindrical cavity of radius R. In this case, advantageously, the electrode can be formed by a filiform element of radius r, with r < R, positioned within said cavity so that it is substantially coaxial thereto. In this way, as a result of the containment of said electrical charges within said cavity and as a result of the conductivity of the air within said cavity, it may be considered that the effective radius, in terms of electrical field generated, of the electrode will be equal to R2, where r<R2 <R With the method according to the invention, it may hence be found that the electrical field generated in the point d of the external environment by an electrode of radius r and subjected to a voltage V will substantially correspond to the electrical field generated in said point d by an electrode of radius R2, with r < R2 < R, subjected to the same voltage V.

Further characteristics and advantages of the invention will emerge more clearly from the description of preferred but non-exclusive embodiments of the device for powder painting according to the invention, illustrated by way of non-limiting example in the annexed plate of drawings, in which: - Figure 1 represents a schematic view of a device for powder painting by corona discharge according to the known art; - Figure 2 represents a schematic view of a first embodiment of a device for powder painting according to the invention; - Figure 3 represents a schematic cross-sectional view of the device illustrated in Figure 2; and - Figure 4 is a schematic cross-sectional view of the device of Figure 2 and illustrates the working principle of a device according to the invention.

With reference to the above figures, the device according to the invention comprises an electrode 14 formed by a very thin conductive wire, for example with radius r of 5-10 pm.

The electrode 14 is advantageously inserted within a tube 15 made of dielectric and insulating material having a radius R greater, and more preferably much greater, than the radius r of the electrode 14. On the tube 15 there is present an opening 16, in this case formed by a thin longitudinal slit that develops along the body of the tube 16.

Preferably, the electrode 14 is fixed, within the tube 15, at either end 17 of the tube by means of two closed terminals, formed, for example, one by a first, insulating, terminal 19 and the other by a second terminal 18 made of conductive material, so that the electrode is kept in a position that is approximately central with respect to the diameter of the tube 15. By connecting the terminal 18 to a voltage generator 20, it is possible to send a voltage to the electrode 14.

With particular reference to Figure 3, analysing the structure of the above device from an electrical standpoint, it is made up of a conductive wire 14, a first dielectric, generally formed <BR> <BR> by air in the cavity 22 within the tube 15, and a second dielectric 21, i. e. , the internal surface of the tube 15 itself.

By appropriately regulating the voltage generator, once a certain voltage is reached, the corona discharge is produced. Since in this case a very thin wire has been used as electrode, in general one much thinner than the normal electrodes used in applications of this type, the corona discharge is produced at lower voltages than the ones normally employed.

Furthermore, the electrical charges thus generated in the air gap 22 between the electrode 14 and the tube 15 render the air itself conductive by ionization and are deposited also on the internal surface of the tube 21. With reference to Figure 4 in fact, within the tube 15 the electrical charges generated 23, represented as spheres, are distributed uniformly in the air 22 between the electrode 14 and the internal wall 21 of the tube 15, said wall serving the purpose of confining the charges, preventing them from coming out. Said electrical charges (ions) render the air 22 contained in the tube 15, between the electrode 14 and the internal wall 21, highly conductive. The result obtained is to create a conductive area by ionization around the electrode 14 that has the same voltage as the wire, but physically expands the radius r thereof as far as the wall 21 of the tube 15. Thanks to this effect, since from the electrical standpoint the electrode 14 is of larger radius, in the limit equal to the radius R of the internal surface 21, an attenuation of the electrical field generated by said electrode 14 is obtained as a function of the ratio of the radiuses, where r is the radius of the original electrode and R2 is the"virtual" radius that is assumed by the electrode as a result of the particular construction of the device according to the invention, namely r < R2 < R.

In fact, if two electrodes of radius r and R2 are considered, with r << R2, i. e., r much smaller than R2, the ratio of the electrical fields generated thereby is equal to: E/IE2 = R21r, where E/is the electrical field generated by the actual electrode of radius r, and Ez is the electrical field generated by the"virtual"electrode of radius R2.

Since then Ez = E, *rlR2, it follows that E2 << E/, and namely, given the same conditions, the electrical field generated by the"virtual"electrode that is obtained using the device according to the invention is much less intense than would be obtained using a device of a known type with a real electrode of the same size r.

Since the effect of the electrical field is reduced considerably, the effects of the Faraday cages and all the other undesirable surface effects described previously are not generated, or at least are reduced considerably.

Preferably, the opening 16, which in the case represented in the attached plate of drawings is formed by a slit made in the tube 15, must be sufficiently thin as not to influence the physical effect of expansion of the diameter of the electrode 14 in a macroscopic way, and hence also as not to affect the flux of the reduced electrical field E2 on the outside of the tube 15.

When the electrical charges generated within the tube 15 have reached a degree of relative density such that the repulsive forces between charges of the same sign may start to prevail, they are obliged by said forces to be dispersed and hence also to pass through the slit 16, to enter the external environment 24 with an acceleration of their own impressed on them by said repulsive forces.

By setting the slit 16 in the proximity (for example at a distance of between 1-4 cm) of the outlet for emission of the powder of any ejection system of a powder-painting apparatus and by orienting the slit 16 towards said outlet, the electrical charges that come out through the slit 16 at high speed appropriately charge, by impact and by capture, the particles of the powder paint.

In order to improve the charge imparted upon the powder, which can be considered proportional to the number of ions emitted and captured, it is possible, without undesirable effects, to apply a double system of charging by setting two devices according to the invention in an area corresponding to the emission outlet, in positions geometrically opposite to one another. In said situation, a double emission of ions is obtained, thus guaranteeing an excellent ratio between the charges generated and the amount of electrical charges transferred to the particles of powder.

The technical solutions illustrated for the powder-painting device of the present invention enable the proposed tasks and purposes to be fully accomplished. In particular, the device according to the invention enables operation with relatively low voltages in so far as it enables the use of a relatively thin electrode. At the same time, the electrical field generated is lower than what it would be with conventional devices given the same conditions.

This enables prevention of the known problems linked to the presence of relatively intense electrical fields, which, as mentioned previously, can generate adverse effects on the quality of the painted product and on the safety of the painting plant as a result of the possible discharges in air that can occur. Furthermore, the possibility of operating at lower voltages enables a saving in the costs both of the plant itself and of the corresponding safety systems.

In practice, the materials used, as well as the contingent dimensions and the shapes, may be any whatsoever according to the requirements and the state of the art.