It is known that the two essential problems outlined above must be faced in the industrial electrostatic painting of all types of end-products, carried out in continuous.

In our case, as an applicative but non-limiting ex- ample, we will illustrate the application of the inven- tion to the painting of metal coils and show how the de- vice claimed is able to overcome the related problems.

It is known that these coils are electrostatically powder painted by means of spray-guns or emitters called rotating cups.

It is also known that an electrostatic spray-gun (53), whose emission terminal is a diffuser, ejects a cone-shaped cloud of powder (51) whose apex faces the emitter itself. Figure 2A.

It is evident that, on taking a section of the emis- sion cone perpendicular to the projection axis, close to the surface to be painted, the density of the cloud of powder differs from the centre towards the edges, area (49), and is lower, area (50), along the centre of the axis as this part is shielded by the diffuser itself.

The powder cone which is deposited on the surface of the material, leaves greater thicknesses at the edges (54), where the emission of the spray-guns (53) crosses, rather than at the projection centre, area (55), Figure 2A.

In the case of emission heads with a longitudinal slit, the opposite happens: the powder has a higher con- centration in the centre of the projection axis and a lower concentration at the edges.

For example, with metal coils having a width of one meter, which are to be painted at a operating speed of 20 meters/minute, with a surface powder coating thick- ness of 50 microns, at least 2 kg of powder deposited per minute are normally necessary.

Current electrostatic systems are only able, theo-

retically, due to the known charge limits of powder, to deposit only 60% of the powder emitted and each has a projection capacity of a weight quantity of 250-300 gr. per minute. In order to paint 20 metres a minute, it is therefore necessary to use a set of at least twenty spray-guns. As known, the powder is conveyed to each spray-gun by means of a pipe (63) called diffuser pipe, connected to a Venturi-type pump (65) fed by compressed air from the pipe (66), at a pressure regulated by means of a reducer valve, with pressure reading applied by a pressure gauge, Figure 4A. The pumps suck the powder through a pipe which draws from an appropriate tank (64) of fluidified powder by air charged through the pipe (62) (fluid bed), Figure 4A.

The flow-rate of each spray-gun must be individually established by regulating the air pressure of its Venturi pump and a further regulation of compressed air, called powder dilution air. If the flow-rate of the powder sent to the spray-gun must be increased, the quantity of air of its Venturi pump must be increased, i. e. we must in- crease its pressure and consequently also increase its discharge rate from the ejector.

As a consequence of the rate increase, there is the formation, in the emission cloud, of preferential strands of air-powder fluid with varying densities and the par-

tial loss of the turbulent movement. The impact of these strands on the surface causes the random deposit of pow- der in relief and, at times, erosion of parts of powder already deposited, with differences in thickness.

The difference in thickness of the powder is also due to: 'The different length of the extent of the diffuser pipes of the powder from the tank to the spray-guns.

The different flow-rates of the compressed air injec- tors of the pumps, for the powder suction.

'Consumption of the internal profiles of the Venturi pumps themselves, due to abrasion of the powder.

Another known problem relates to having to place the spray-guns on a line which is such as to cover, with pow- der emission cones, the total width of the arrival sur- face.

As the emission cones of each spray-gun are forced to cross, in order not to leave uncovered surface ele- ments, area (56) Figure 2A, it is evident that, in these points, the surface is covered by a double jet with a considerable increase in the powder deposited.

The deposition is influenced by the electrostatic charge supplied to the powder due to corona discharge (effect of the emission of electrons, from an electrode connected to a high voltage generator, which, by attach-

ing themselves to the atoms of the molecules of surround- ing air make them electrically charged, known as ions, instead of neutral). With normal electrodes used, the fraction of ionic current which charges the particles of powder is estimated as being 0.5% of the total.

The low efficiency ratio is due to the geometry of the emission electrode which is generally positioned co- axially to the centre, in the terminal part, of the out- let pipe of the powder. Under these geometrical condi- tions, the current, discharged from the electrode, is limited, as known for repulsion, by the electric charge with the same sign acquired by the powder which flows around it. The only way of enhancing the charge is to in- crease the voltage of the electrode to raise the quantity of current discharged. As known, a rise in the voltage causes a harmful increase in the electric field near the surface to be painted. The increase in the electric field induces undesired effects on the surface covered by the powder deposited, once baked and polymerized such as: a) orange-peel surface effect, b) surface with a pin-holes effect, c) separation of the particle-size of the powder with differences in colour.

The geometrical shape of the electric field, gener- ated by the electrodes of the spray-guns (53) is repre-

sented by a number of lines of force (57), proportional to the intensity of the field, Figure 2B, which extend from the electrodes (52) and terminate in general points of the surface.

The projected image of the lines of force (57) of the surface electric fields, has a certain value on the vertical of the individual point-electrode increasing where the fields cross with each other (59), Figure 2B.

As is known in physics, when electrostatically charged powder is deposited, it follows the lines of force of the electric field and consequently the thick- ness of the deposit has in turn maximums and minimums.

Coming back to the example of the painting of coils at twenty meters per minute, at least twenty spray-guns should be used, with twenty Venturi pumps, forty pressure regulators, forty reading manometers, ten double elec- tronic air pressure control centers (61), Figure 4A, pipes (66) and a high electric voltage of the spray-guns, cables (60).

The running of a similar system becomes uncertain and difficult due to the number of regulation parameters to be established, for each single element belonging to the system itself.

As far as painting with rotating cups is concerned,

the same problems apply, except for the advantage of us- ing a lower number of projectors, thanks to their higher flow-rate of powder.

The aim of the invention is to overcome the solve the problems described above, achieving the following ob- jectives: a) Reduction to one or two projectors installed for painting, b) Reduction in the number of Venturi diffuser pumps of the powder itself, c) Reduction in the diffuser regulation systems of the powder, d) Linear discharge and a uniform density of the powder to obtain emission without cross-points, e) Geometry of the electrode so that the electric field near the surface is uniform and without discontinuity points, f) Emission of electrons from the electrode to have a uniform and non-dispersed formation of ions for the optimum charging of the powder, g) Reduction in the number of electrostatic generators necessary for charging the powder, h) Increasing the surface painted within the time unit, thus increasing production with a considerable eco- nomic advantage.

The device of the invention consists of a rectangu- lar tank (16) Figure 1A represented in section, containing two porous septa inside (7) and (13) which di- vide it into three superimposed chambers (4), (5) and (6). The rubber socket (11) on one side of the container (19) serves for the inlet pipe (41) of the powder from a general feeding tank (46), aspirated by means of a single Venturi pump (44) with a high flow-rate, Figure 4B.

The compressed air is fed into the chambers (6) and (4) by means of inner pipes (24) and (10), holed (14) and connected at the T-shaped socket (43), with pipes (42), (45) and with the air intake pipe (40), Figure 4B. The air passes through the porous septa (7), (13) and fluidizes the powder which enters the chamber (5) through the pipe (41), Figure 4B. The septum (15) verti- cally separates the chamber (5) into two sub-chambers (21) and (20). The septum (15) is at a suitable distance from the porous septum (7), in order to allow the enter- ing powder (which once fluidified behaves as a liquid) to occupy the two chambers (21) and (20) acting as communi- cating vessels, Figures 1A and 1B.

The chamber (20), which, after a period of time, must integrate the quantity of powder sent through the pump (44), has a larger volume than the chamber (21).

In order to emit the powder from this tank (16), a Venturi-shaped ejection profile of the powder (17) has been adopted, partly situated in the lid (12) and partly in the block (18), whose length is slightly higher than that of the metallic coil to be painted. Two superimposed longitudinal slits (19) and (3) are situated in this ejector block (18) by means of the septum (8) for the projection of the powder. The emission of the powder takes place both as a result of the pressure of the air sent through the porous septa (7) and (13) into the cham- ber (20), and also by means of the diffused air pressure of the Venturi pump (44) into the same chamber (20) which also communicates with the chamber (21) where the powder is pushed out through the slits (19) and (3), Figure 1A, to paint the coil.

The Venturi-shaped ejection profile of the powder (17) is constant for the whole length of the device (16), which is connected by means of the pipe (41) to a single diffuser pipe of the powder (44) fed by means of the air entering the pipe (49), to the air pipe (47) for the fluid bed of the powder tank and, to the pipe (48), for the porous septa of the emitter (16). This configuration advantageously allows the number and time of regulations to be effected by the operators to be reduced and simpli- fies the optimum set-up of the painting device of the in-

vention, as it can be clearly seen from a comparison of figures 4A and 4B, which compares a traditional spray-gun system with the device of the invention.

Above the powder outlets (19) and (3) there is the double electrostatic charge system of the powder (1) and (2).

This consists of a metallic pipe (1), with reference to Figure 3A, having a length which is slightly higher than that of the emitter with a slit (37) figure 3B, in section, longitudinal with an opening of about 100°.

Two plugs, made of an electrically insulating plas- tic material, (33) and (34) are inserted at the ends of the milled pipe (1) figure 3A.

At the end of the plug (33) there is a screw connec- tion (31) for inserting the high voltage cable (39), com- ing from the voltage multiplier bridge (36), fed to the electronic control center (40), figure 3C.

The high voltage cable (39) is regulated so as to make contact with the plate (38) to which an electrically conductive metal wire (2) acting as electrode, is at- tached, which is connected to the other end plug (34) where it is kept under stress by the spring (35) with the possibility of regulating the stress by means of the screw (32).

As the electrode in this case is a wire, and there- fore has a continuous geometry, the electric field, dashed lines (58), Figure 2C, generated thereby, close to the surface of the coil, also has the advantage of having the same continuity and uniformity, eliminating the maximum and minimum points which were formed, as specified above, with the use of several projectors.

Another advantage lies in the use of the milled pipe (1) which acts as a shield and electrostatic lens for fo- calizing the electric charges generated by the electrode wire (2), Figure 3D.

In order to increase the painting velocity of the coils, both the quantity of powder emitted from the pro- jector and the electrode current must be increased, as there would otherwise be a large quantity of powder which could not be sufficiently charged for adhering to the coil.

In the case of a traditional charge system, it is known that the quantity of current absorbed by the powder emitted, with respect to the total amount, is about 0.5%.

It can therefore be deduced that there is a very high dispersion of ions generated by the corona discharge and only a minimum fraction of these contributes to charging the powder.

The device of the invention has been designed so

that the emission of ions (68) takes place in a single direction through the slit (37) towards the flow of pow- der (69) projected from the emitter, decreasing the dis- persion, Figure 3D. The electrons emitted by the wire (2) ionize the molecules of air present in the space between the walls (30) of the pipe (1) and the wire it- self, creating an electric charge of the same sign. The pipe is immersed in the electric field due to the voltage present on the wire and its electric potential is conse- quently slightly lower than that of the feeding of the wire itself, due to its minimum distance from the wire.

The electric charge, generated inside the pipe (1) by the electrode (2), is attracted towards the walls (30) as their electric potential is lower, Figure 4D. As the current which can be absorbed by the walls (30), is limited, there is an accumulation of electric charges (ions) of the same electric sign near the wall, which re- ject the new charges which are continuously created due to corona discharge, Figure 4D.

The new charges (68) therefore only have the possi- bility of exiting from the slit (37) situated in the di- rection of the ejection outlet of the powder (69), cross- ing the flow and consequently electrically charging it.

The edges (67) of the slit are at a voltage of the same electric sign as the ions generated by the electrode, but

as a result of their edges, due to known physical ef- fects, they produce a strong electric field and conse- quently have a repulsion effect on the same ions, forcing them to be discharged in a fine linear beam at the centre of the slit (37), in the direction of the powder without being dispersed in the environment, Figure 3D.

Moreover, as the discharge of the powder (69) takes place through two continuous slits (3) and (19) depending on the width of the coil, there is no discontinuity on the surface due to the crossed emission of the spray- guns, Figure 2C.

A further advantage lies in the installation of the device of the invention on a plane attached to it (25), also connected to another supporting planee (27) by means of four anti-vibration supports (28) and (26).

A pneumatic or mechanical vibrator (29) is installed on the surface (25), which guarantees the necessary vi- bration for obtaining the best results for a clean emis- sion and uniform deposit, Figure 1B.

It can be clearly seen how the device of the inven- tion, although extremely simple in its construction and running, provides a considerable improvement to the sur- face finishing, a saving in the installation and operat- ing costs, but at the same time also has the possibility of increasing the production within the time unit, thanks

to its high emission capacity and electric charge of the powder.

The device of the invention has been illustrated and described in one of its preferred embodiments but numer- ous constructive and applicative variations can obviously be applied, which are included in the protective scope of the present industrial invention patent.