WO2001085353A1

WO2001085353A1 - Plant for electrostatic painting with a venturi nozzle

Info

Publication number: WO2001085353A1
Application number: PCT/EP2001/005495
Authority: WO
Inventors: Paolo Checcucci; Giovanni Bortolato
Original assignee: Paolo Checcucci; Giovanni Bortolato
Priority date: 2000-05-10
Filing date: 2001-05-10
Publication date: 2001-11-15
Also published as: ITMI20001023A1; EP1280608A1; AU2001265959A1; IT1317486B1; ITMI20001023A0

Abstract

An electrostatic powder coating system comprises a powder ejector (3) and an electrode (2) for electrical charging of the powder. The ejector (3) has a linear slit-like opening and comprises a venturi through just upstream of the opening; the electrode (2) is placed along the ejector over its whole length.

Description

PLANT FOR ELECTROSTATIC PAINTING WITH A VENTURI NOZZLE

The present invention relates to plant or system for electrostatic spraying in general and, particularly, for metallic coils coating, in order to increase both the production of finished product in unit time, and uniformity of the coated layer deposited on the surface. It is known how in the industrial electrostatic coating, in continuos process, regarding every manufactured article, there are the two above essential problems to solve. In our case, as application but not limitative example, we want to show the appliance of the invention to the coils coating made in continuous cycle and how to solve the problems concerning this application. It is well known that such coils, till now, are electrostatically powder coated with guns or rotating cups. It is also known as an electrostatic gun, having a diverting conical diffuser as emission terminal, ejects a cloud of powder cone shaped, with the apex towards to the same terminal.

It is clear that the cloud density of powder, taking a section of the emission cone perpendicular to the ejection axis, near the surface to be coated, changes from the axis center to the edges, being certainly more thin along the axis center because this part is shielded by the diffuser itself. It is also clear as the powder cone deposited on the material surface covers especially the edges, where the emission clouds of the guns cross themselves rather than to the projection center.

In case of powder spraying through a longitudinal gap it happens the opposite: the powder has major concentration to the axis center respect to the edges.

Furthermore, in order to coat large surfaces in the unity time, as required for coils painting, with a powder coated around 50 micron, as requested by quality specs., it is necessary to use batteries of several guns.

Usually, for instance, if we want to paint metallic coils of one meter width at operating speed of 20 meters/min., with a surface powder thickness of 50 micron, at least 2 kg deposited powder is necessary. The known electrostatic systems are able, in theory, due to the well-known charging limits of the emission powder in big quantities, to deposit 60% of the ejected powder and each of them has an ejection capacity of 250-300gr/min. Therefore, it needs to use battery of at least 20 guns in order to paint with results requested by quality specs.

As we know, during the electrostatic coating process, the powder is feeded to each gun through a tube, connected to a Venturi pump feeded with compressed air by the tube, at pressure regulated through a reducer valve, with a manometer.

The pump sucks the powder, to be sent to the guns, through a tube which aspires in a suitable container of fluidized powder (fluidized bed). The powder emitted by each gun must be fixed separately through the air pressure regulation of the own Venturi pump and of a further compressed air regulation, called dilution, brought to the gun or to the exit of the Venturi pump in order to increase, within certain limits, the powder mixing and to decrease its density.

If we increase the quantity powder sent to the gun, we must increase the air pressure of its own Venturi pump. As it is known, by increasing the air pressure of Venturi pump , we also increase the speed of the gun powder cloud emission.

The consequence of the speed increase is the formation, in the emission cloud, of thin threads of air-powder fluid with different density and the partial loss of the turbulent motion, due to the diffuser, which assured a certain minimum air-powder mixture.

The impact of these thin threads on the surface to be coated brings to the formation of powder excrescencies on it and, some times, the erosion of powder parts already deposited. This regulations involve difference emission powder shape of every guns and quantity of powder ejected. These problems are due to a series of elements as: 1. the different tubes length of powder flow to the guns, due to their different position 2. the latch-shape of the route each tube must cover to reach the gun to which it is connected

3. the suction tubes position of Venturi pumps in fluidized powder container.

4. the air compressed injectors of the pumps for powder suction.

5. the erosion due to the powder scratching in the inside injector profiles of the Venturi pumps.

Having the guns, in battery, a different powder ejection, the part of surface coated by each of them has, in its turn, a different thickness.

Another known problem is to place the guns on such a line to cover, with cones of powder emission, the total width of impact surface. In order to avoid surface uncovered, the emission cones of each gun must, necessarily, be superimposed. It is clear that, in those points the surface is covered by a double jet with a remarkable increase of deposited powder. Other elements which influence the deposition are: - the quantity of electrostatic charge given to the powder due to the Corona effect (known physics effect of electrons emission from an electrode connected to a high voltage generator which by itself attaching to the atoms of the surrounding air molecules make them, from neutral, electrically charged and therefore called ions).

- the quantity of the electrical charge absorbed by a powder particle is proportional to the number of ions attaching to its surface. The number of ions generated depend on the emission current of the electrode which also depends on the applied voltage and its distance from the surface to be coated. With the normal electrode systems used, the fraction of emission current which charges the powder particles is estimated at 0.5% of the total. The low efficiency ratio is due to the emission electrode geometry which is, in general, placed coaxial to the center, in the final part of the exit powder tube. In these geometrical conditions, the current delivered from the electrode is limited, as we know, by the electric charge accumulated by the powder running around. In fact, as the powder starts charging electrostatically, the acquired charge which has the same sign of the ions generated by the electrode, the known physical effect of repulsion among the electrical charges of the same sign begins .

In this way the repulsion ends by dispersing in the surroundigs and spreading a large quantity of ions which, otherwise, could have charged the powder. For its geometrical shape, the electrode has not the possibility of working as a lens able to focalize or address the charges, largely, on the powder. To maintain the necessary charge ratio per kg. of emission powder (as advised), the mentioned systems must increase the number of molecules of ionized air able to charge the powder particles.

The only way to obtain it is to increase the electrode voltage in order to raise, in its turn, the quantity of electrons emitted. As we know,by increasing the voltage we have a prejudicial damaging increase of the electric field near the surface to be coated while increasing the current we have ions dispersion. The increase of the electric field and the dispersion of these charges gives undesirable effects on the surface covered by deposited powder, when cooking and polymerizing like: a. orange-peel effect on the coated surface b. pin holes effect on the coated surface c. granulometric separation of the powder with surface color differences.

The geometrical shape of the electric field, produced by the electrodes of the corona effect guns, closeness to the surfaces to be coated has a great importance in order to obtain a uniform covering.

Such shape is represented by a number of force lines proportional to the intensity of the field departing from the electrodes and ending on generic points of the surface. We can represent the electrodes of guns battery as electric charges points of different intensity, placed on the same line and spaced between them like the distances which lie between the guns . The projected image of the force lines of the electric field on the material surface to be coated, is immediately more intense on the vertical of the single electrode-point and it decreasing between them.

In these conditions the lines-force of the electric fields, produced by the single electrode on the surface, have some discontinuity in the overlap zones, where their intensity is added creating many zones with maxima and minima. In these zones, as it is known by physics, the powder paint particles, electrostatically charged, when depositing, follow the lines of force of the electric field and therefore the deposit thickness, on its turn, will have maxima and minima.

To resume the example of coils coating at 20 meters/min., according to prior art we should employ at least 20 guns, using 20 Venturi pumps, 40 pressure regulators, 40 manometers, 20 high-voltage electronic generators for the electrodes of the same guns.

To manage such a system it is hazardous and problematic due to the number of regulation parameters which must agree, for each single element belonging to the very system, in order to obtain the quality requested by the rules.

As regards to the coating of rotating cups the problems remain the same but with the advantage of using an inferior number of projectors thanks to their major powder capacity. The invention aims at overcoming and solving the problems above mentioned by achieving the following aims : a. use of a single projector b. cut in the number of Venturi pumps c. cut in regulation systems of powder capacity d. powder issued in linear form and at a uniform density in order to obtain an issue without cross-points emission. e. to keep constant the tank pressure also when the pressure of delivery air increases f. checking of electric field near the surface in order to make it uniform and without discontinuity points g. checking and focalization of the electrons emission of the electrode in order to have a uniform ions forming to charge the powder in an optimal way h. checking of current charge quantity without increasing the electrode voltage and therefore the intensity of the electric field near the surface i. reduction of the number of electrostatic generators necessary to charge the powder j . to increase the coated surface in the unit time in order to increase the production with a great economic advantage. In view of these purposes it is sought to provide in accordance with the present invention an electrostatic powder coating system characterized by comprising a powder ejector with linear outlet and laminar flow throughout a profile of Venturi type with planar geometry and an electrode for electrical charging of the powder, the electrode being placed along the ejector for whole length. To clarify the explanation of the innovative principles of the present invention and its advantages compared with the prior art there are described below with the aid of the annexed drawings possible embodiment thereof by way of non- limiting example applying said principles. In the drawings: Figure 1 shows an embodiment of a plant in accordance with the prior art;

Figure 2 shows an embodiment of a plant in accordance with the present invention;

Figure 3 shows a transversal section of a element of the plant according to the present invention. Figure 4 shows schematically a plant operation view.

Figure 5 shows schematically a front view of a element of the plant according to the invention.

Figure 6 shows schematically a front view of an other element of the plant according to the invention. Figure 7 shows a section takes along the line VII-VII of the figure 6.

Figure 8 shows schematically a possible connection of the element of figure 6. Figure 9 shows schematically an other plant operation view. Figures 10 and 11 show schematically transversal sections of an emission head during the operation. Describing this invention we will show an appliance, as previously said, related to the metallic strips (coils) coating, without cutting out the numerous intrinsic possibilities of using for every other type of application. With references to figures, figure 1 show a typical plant of prior art. In the figure 1 we reported an example of the necessary connections for only ten guns, where the electric cables are well-evidenced 29, the air-compressed delivery tubes 62 to the Venturi pumps 28 placed on the fluidized container 33, the powder delivery tubes 63 to the guns 53, the fluidized bed air-pipe 46 of the powder container 33, the control rack 61 of air-pressure and electric feeding of the guns. The figure 1 shows the peripheral zones 49 of greater density of powder and the central zones 50 of smaller density of powder. The zones of superimposition can be also observed. The already above mentioned very complex structure is clear.

Figure 2 shows a plant according to the present invention. The plant or system comprises a rectangular container 16. An input connection 11 on the upper cap 19 needs for the powder inspiring tube 27 from a general feeding container 25, sucked by the only Venturi pump 26 of big capacity. The Venturi pump is know prior art pump. Therefore, it is not showed or disclosed in detail.

On the cap 19 a tube 47 is inserted to its upper end where there is an escape valve 48 which regulates the inside container pressure due to the inspiring air of powder, in order to work at a constant pressure.

As figure 3 shows, at the inside of the container 16 there is a porous media or septum 7 which divides the container in two laid upon chambers or zones 5 and 6.

The compressed air connection 22 inspires the air through the internal connection tube 9 to the distribution tube 10 and through the holes 14 in the chamber 6, which throughout the porous media 7 fluidizes again the powder brought in the chamber 5 through the tube 27 for carrying out a fluidized bed of the powder on the porous septum 7. A septum or vertical wall 15 is suitably spaced on the porous media 7 in order to allow the inspired powder (which when it is fluidized it behaves as a liquid) to occupy the two chambers 21 and 20 as they were communicating vessels. The function of the septum 15 is also to remove turbulence in the chamber 21 due to the air inspiring of the powder, under pressure, of Venturi pump 26 to the chamber 20. This chamber, having to integrate, in the time, the quantity of powder delivered through the pump 26, has a volume greater than the chamber 21.

In order to issue the powder from this tank 16 we have chosen a linear ejection profile 17 for ejecting the powder, with a width lightly superior to that of metallic coil (or surface) 80 to be coated and moved by a conveyor 100. In other words, the ejector is extended tranversally the moving direction of the conveyor and has width at least equal to the useful width of the conveyor. Such profile has, in section, a Venturi ejector shape.

In this ejector 17 a suction and mixing chamber 30 has been built in order to pump the powder by the tubes 32. The suction takes place both for the depression due to the air ejected, at high speed, from the injectors 13 to the chamber 30, and for the air pressure of Venturi pump 26 to the chamber 20 which is conveyed also to the chamber 21 where the powder is. The tubes 32 supply to the ejector drawing the powder from the fluized bed on the porous septum 7. The sucking chamber 30 on the one side is connected to a conduit 31 of powder flow and on the other is closed by an air compressed distributor, sealed and parallelepiped shaped 12, which brings a right number of air injectors 13, spaced in such a way that the sucked air in the chamber 30 is mixed for turbulence so that the powder delivery from the conduit 31 can take place in laminar shape 67 (as schematically shown in figure 4) and do not show pratically density changes which should cause thickness differences on the surface 80. The distributor 12 is fed by air compressed connections 39 and 41 placed to the emitter sides, by air flow tube 40 joined in a T shaped connector 43 and from this one to further two tubes 42 and to the mentioned connections 45, as figure 2 shows. The advantage of such configuration is further on pointed out that the powder sucking tubes 32 are inside: all of the same length, same route of powder flow (vertical) and extremely shorter than those normally used (few centimeters against some meters).

A further advantage is to use air pressure of sucking powder lower than those normally used in the guns, utilizing the pressure exercised by the air flow of Venturi pump 26 with a remarkable air compressed save. The Venturi profile shape, of powder ejection 17 is constant throughout its length; the sucking air injectors 13, through their internal distributor 12 are all connected to the same pressure regulation valve 60, the emitter is connected to only one powder delivery pump 26 with the tube 27; the air tube 46 for the fluidized bed of the powder tank and, the tube 68 for the fluidized bed of the emitter

17.

This reduces the number and the time of the regulations to be made by the operators and furthermore it makes the optimal setting up of the invention for coating easy.

As figures 3 e 5 show, placed upon the powder exit mouth 3 there is the powder electrostatic charge system 1 and 2. This system is extended along mouth of powder emitter. As better figures 6 and 7 show, this electrostatic charge system consists advantageously of a metallic tube 1 of a length lightly superior to that of the emitter and having a longitudinal gap 37 in one side and with an opening of about 100°. Two plastics insulating caps 33 and 34 are connected to the extremity of the gap tube 1.

The cap 33 has at the end a connection screw 8 where it is connected the high voltage cable coming from voltage multiplier bridge 36, fed by the electronic rack 24 (Figure 8) .

The high voltage cable 4 is regulated in such a way to make contact with the link 38 to which the electric wire 2 is connected, acting as electrode, joining to the other terminal cap 34 where it is strained by a spring 35 placed inside the terminal 34 with the possibility of regulating the wire tension through the screw 23.

Being, in this case, the electrode a wire, therefore at continuous geometry, also the electric field 58 generated near the coil surface, represented by dashed lines , has the same continuity and uniformity, eliminating the points of maxima and minima which could show, as previously described, using more projectors. This is showed schematically in figure 9. An other advantage is given using the tube 1 which acts as screen and its gap as electrostatic lens in order to focalize the electric charges generated by the wire electrode 2 (figure 10).

As described, the current delivered by the electrode is, in the traditional systems with guns, directly proportional to its feeding voltage and inversely proportional to its distance from the surface to coat.

If we want to increase the coating speed of the coil, we must increase both the powder quantity delivered from the system and the electrode voltage otherwise we should have a big quantity of powder incapable of sufficiently charging itself to adhere to the coil.

In case of a traditional charge system, we know that the quantity of current absorbed by the delivered powder, respect to the total is about. 0.5%. This means that there is a strong dispersion of ions generated by the Corona effect and only a little part of them contributes to charge the powder . In the invention we have worked in such a way that the ions emission 64 takes place in only one direction throughout a gap 37 to the powder flow projected by the emitter, decreasing the dispersion, as shown in figure 10. The electrons emitted by the wire 2 ionize the air molecules presented in the space between the tube walls and the wire, creating electric charges of the same sign. The tube is connected to the voltage generator ground through a variable resistor 66 of chosen value in order to absorb more or less one part of the current generated by the electrode wire. The tube is also immersed in the electric field due to the wire voltage and therefore, due to the distance from the wire, it is at an electric voltage lower than that of wire feeding.

The electric charges, generated inside the tube 1 by the electrode 2, are attracted towards the walls 44 as they are at more low electric voltage.

As the current absorbed by the walls 44 is limited by the resistor 66, near the wall we shall have a space charge of electric charges (ions) of the same electric sign which reject the new charges continuously created by the Corona effect .

The new charges 64 have, therefore, the only possibility of going out from the gap 37 placed in the direction of the powder 69 ejection gap, crossing its flow and then electrically charging it.

The edges 65 of the gap have a voltage of the same electric sign of the ions generated by the electrode, but due to their corners, as for the known physics effects, they produce a strong electric field so they have a repulsion effect on the same ions, obliging them to outflow to the center of the gap 37 in a fine linear bundle. Changing the value of the resistor 66 to which the tube is connected we can control, being the voltage electrode and distance from the surface equal, the quantity of charged current without varying also the electric field.

As shown in figure 11, to increase further on the quantity of ions 64 having the chance to attack to the powder 69 a metal strip 18 connected to the ground 70 of the voltage generator, has been placed under the emission gap 3. Advantageusly, the strip 18 is grounded by a variable resistor 81 in order to control field distorsion. Since the distance of this metal strip from the electrode 2 is very inferior to that between the surface to be coated and the wire electrode, the electric field generated will be divided in two parts: one very intense on the metal strip 18 and one very weak on the surface of the metal coil, reaching so the prefixed aim to have not intense electric fields on the surface of the coil and to avoid the undesirable effects on the coating as previously described. The generated ions 64, moreover, following the force lines of the field, (the shape of the electric field is, in this case, equal to that of the lines of ions 64, figure 11), cross, in order to reach the link at more low potential, almost totally the powder flow 69, increasing in this way the probability of powder charge, without dispersion in the surroundings .

It is clear that the invention, even being its construction and its management very simply, produces a remarkable improvement to the superficial finish, a save of installation and use costs, and it is also possible to increase, thanks to its big capacity of powder emission, the production in the time unit.

The invention has been illustrated and described in an chosen form of realization and application, but it is well understood that many constructive variants can be produced without going off the protection ambit of the present patent of industrial invention.

For example, the powder emission can be vertical down rather than orizontal, as easy to be thought by one expert in the art, for painting surfaces conveyed in orizontal.

The conveyor 100, as well as comprising a belt, can be of any kind of prior art.

Claims

1. Electrostatic powder coating system characterized by comprising a powder ejector with linear outlet and laminar flow throughout a profile of Venturi type with planar geometry and an electrode for electrical charging of the powder, the electrode being placed along the ejector for whole length.

2. System as claimed in claim 1, characterized in that the electrode is shielded for its whole length by a tube with a gap for the electric charges emission parallel to the ejector outlet, the edges of the gap carrying out an electrostatic lens to focalize charges on the powder coming out from the ejector.

3. System as claimed in claim 2, characterized in that the tube is axially rotable for orienting the gap of the tube respect to powder ejector.

4. System as claimed in claim 2, characterized in that the gap has an opening of about 100°.

5. System as claimed in claim 2, characterized in that the tube is connected to electrical ground of an high voltage generator supplying the electrode.

6. System as claimed in claim 5, characterized in that the tube is connected to the ground by an resistor adjustable in order to control emission current.

7. System as claimed in claim 1, characterized in that the electrode is a conductive wire.

8. System as claimed in claim 1, characterized in that the electrode is placed directly over the powder ejector.

9. System as claimed in claim 2, characterized in that a ground surface placed on side of the ejector opposed to the electrode in order to deflect partly the electric field due to the electrode voltage.

10. System as claimed in claim 9, characterized in that the ground surface comprises a metal strip extended parallel along the ejector and grounded.

11. System as claimed in claim 10, characterized in that the strip is connected to the ground by a resistor adjustable in order to control the deflection of the electric field.

12. System as claimed in claim 10, characterized in that it comprises a conveyor for moving surfaces to be painted in front of the ejector, the ejector being extended tranversally the moving direction and having width at least equal to the useful width of the conveyor.

13. System as claimed in claim 1, characterized in that the powder is supplyed to the ejector through a chamber of turbulence absorption.

14. System as claimed in claim 13, characterized in that the chamber is subdivided by a porous septum in two parts superimposed, in the lower part compressed air is supplyed and in the upper part powder is supplyed for carrying out a fluidized bed of the powder on the septum, tubes for supplying the ejector drawing the powder from the said fluized bed.

15. System as claimed in claim 14, characterized in that the said upper part is in turn subdivided partially in two chamber by a vertical wall, opened in the lower end, in one of the chambers being placed a inlet of the powder and in other of the chambers being placed said tubes for supplying the ejector, the powder flowing from one to other chamber through the lower opened end of the wall by means of communicating vessels effect.

16. System as claimed in claim 1, characterized in that the ejector has, behind the Venturi profile, a plurality of air complainer ejectors, the air complainer ejectors introducing compressed air in the back of the Venturi profile for the powder suction from the inlet of the powder ejector.

17. System as claimed in claim 16, characterized in that a single feeding air distributor supplies the plurality of air complainer ejectors.

18. System as claimed in claims 14 and 16, characterized in that a said tube for supplying the powder ejector from the fluized bed exists for each air complainer ejector.

19. System as claimed in claim 18, characterized in that the powder sucked by all tubes is mixed in a expansion chamber in the back of the Venturi profile.

20. System as claimed in claim 13, characterized in that the powder is supplied to the chamber of turbulence absorption by means of a Venturi pump.

21. System as claimed in claim 13, characterized in that the chamber of turbulence absorption comprises a valve to adjust the air pressure in the chamber.