WO2003055609A1

WO2003055609A1 - Device for electrostatic painting of metallic strips with increase deposition efficiency

Info

Publication number: WO2003055609A1
Application number: PCT/EP2002/000116
Authority: WO
Inventors: Giovanni Bortolato
Original assignee: Lasa Impianti S.R.L.
Priority date: 2002-01-02
Filing date: 2002-01-02
Publication date: 2003-07-10
Also published as: EP1461160A1; ATE290930T1; EP1461160B1; DE60203326D1; AU2002237262A1; ES2236484T3

Abstract

A device for painting with electrostatic powder metallic strips, comprises a painting system (4) which emits the powder toward the strip (1). Strip tension means (22) are positioned in such a manner that the strip (1) forms an acute angle. Two suction means (16, 17) are located along the strip planes (18, 19) coming out of the angle. The suction means (16, 17) suck off parallel to said planes and the painting system (4) emits perpendicular to the angle formed by the planes.

Description

DEVICE FOR ELECTROSTATIC PAINTING OF METALLIC STRIPS WITH INCREASE DEPOSITION EFFICIENCY

The present invention relates to a device for exposing a metallic strip to electrostatic painting and suction of the excess powder projected and enabling increased production velocity and- finish quality. 0 It is known that in industrial painting of metallic strip performed continuously there are some unsolved problems to be faced one of which is the increase in strip running velocity so as to increase the quantity of product finished in a time unit, a problem concerning the solution proposed 5 by the present invention.

It is known that such strips are painted by using electrostatic guns or other types of emitter called rotating cups all positioned perpendicularly to the flat surface of the strip. It is also known that an electrostatic charge is 0 imparted to the powder to allow adhesion thereof to the support, in our case the strip, by the force of electrostatic attraction termed image forces until the support is sent into a baking oven where the powder at the appropriate temperature is polymerised. 5 To paint large surfaces in the time unit with a powder cover of at least 50 microns as required by the quality specifications it is necessary to use batteries of several projectors. Normally for example with metallic strips one meter wide which it is wished to paint at a velocity of 20 meters per minute with a thickness of deposited powder of 50 microns at least 2 kg of deposited powder per minute are necessary. Present day electrostatic systems theoretically manage to deposit 60% of the powder emitted and have an average projection capability of a quantity by weight of 250 g per minute each.

Therefore, to paint with the results called for by the quality specifications it is necessary to use a battery of at least twenty guns.

The excess powder which is not deposited on the strip surface is sucked through an appropriate hood and then fed back into the paint circle after sifting. The powder is taken to each gun through a delivery tube connected to a venturi pump fed with compressed air by means of an adjustable pressure reducing valve with pressure readable on a pressure gauge.

The pumps suck proportionately to the supplied air pressure a certain quantity of powder to be sent to the guns through a tube drawing in a tank of fluidised powder.

The flow of each gun must be set individually by regulating the air pressure of its venturi pump and additional compressed air termed 'dilution air' . It is clear that to increase the painted surface area in the time unit we must increase the number of electrostatic guns or increase the flow of powder to the guns in the time unit. Deposit on the strip surface of the electrostatically charged powder is subject basically to two types of forces, to wit: 1. the aerodynamic forces of the particles projected by the venturi pump air flow pressure; increasing air pressure increases the amount of powder and also the powder emission velocity from the gun, and 2. the electrostatic forces resulting from the electrical fields of the gun electrodes in the space between them and the surface to be painted, and

3. the electrostatic image forces of attraction of the charged particles near the strip surface. The amount of charge absorbed by the powder particles decisively influences deposit on the strip surface. The powder is charged by the corona effect (known physical effect of emission of electrons from an electrode connected to a high voltage generator which in attaching to the surrounding air molecules makes them, from neutral, electrically charged negative charges and in turn they adhere to particular zones of the surface of the powder grains known as electronegative affinity) .

The amount of charge imparted to the powder depends on several parameters, to wit: the electrode emission current which in turn depends on the voltage applied and the distance thereof from the painting surface, and the density of the air & powder mixture emitted; the denser it is the denser is the dispersion of the electrical charges and the smaller the quantity of powder charged electrostatically. The increase in the electrode current under specific conditions allows charging a larger quantity of powder emitted thus approaching the 60% theoretical deposit.

Now we can examine the phenomena triggered upon increase of the number of electrostatic guns and the increase in powder flow. 1. The increase in the number of guns and thus of the electrodes (one for each gun) increases the intensity of the electrical field due to the voltage of the electrodes in a manner approximately proportional to their number near the surface of the strip. The increase in the intensity of the electrical field near the surface to be painted triggers two other different phenomena, to wit: a. one is the known counter corona effect due to breakage of the dielectric strength of the air between the first layer of powder particles deposited and the surface of the material to be painted. This effect generates positive air ions which when returning towards the gun electrode fed with negative voltage neutralize the powder charges , the latter being negative also, so that the powder, not being charged electrically, cannot adhere to the surface. b. The second is a repulsion effect between charges with the same sign. The powder particles charged negatively find near the surface an electrical field generated both by the particles already deposited and by the gun electrodes with the same sign which repels them, slowing their deposit velocity and thus preventing an effective covering of the material. c. The velocity of output of the powders upon increase of the air pressure can be such as to cause erosion of the layer deposited due to the friction effect. The general purpose of the present invention is to solve the above mentioned problems . In particular, the positioning and suction device which is the object of the present invention also achieves the following purposes: a. reduction of the electrical field on the surface to be painted, b. reduction for the same emission of the powder jet impact force on the surface to be painted, and c. enablement of increasing air pressure and outlet velocity without causing erosion. 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 the normal positioning of a metallic strip and electrostatic guns in a painting plant.

- Figure 2 shows diagrammatically the field intensity in the device of figure 1.

- Figure 3 shows the trend of the powder near the surface in the device of figure 1. - Figure 4 shows graphically the repulsion effects in the device of figure 1.

- Figure 5 shows a possible embodiment of a device of the present invention.

- Figure 6 shows the tendency of the gun electrical field lines in the device of figure 5.

With references to figures, in accordance with the prior art, figure 1 shows the normal positioning of the metallic strip (1) stretched by the tension rollers (2) and (3) with the electrostatic guns (4) in painting position perpendicular to the surface.

By feeding the gun electrode (5) by a high voltage generator (6) an electrical field E is established between the guns and the strip surface. Table 1 Fig. B show the so- called lines of force (7) with broken lines in geometrical form.

Figure 2 shows diagrammatically the field intensity near the strip surface with the darker segment ( 8 ) occupying a portion of the strip surface approximately equal to that of the powder emission cone from the gun.

As compressed air is sent to the venturi pumps of the guns the latter will begin delivering powder with a trend near the surface shown in figure 3 in plan view. As may be seen the trend of projection of the powder (9) is characterized by zones (10) where the rebound of the particles thereof projected towards the surface causes rotary motion thereof, which is accelerated on the edges of the strip because of the air suction of the hood (11) to recover the powder particles which were not deposited. Because of the powder friction effect this acceleration can cause considerable thickness differences on the strip edges due to erosion as explained above.

Due to the sum of the individual electrical fields generated by the gun electrodes the very intense electrical field on the strip surface in the powder deposit zone has the described effect of having a repulsion force towards the powder particles which are to be deposited on the surface. This force tends to accelerate the powder with a direction contrary to that of their aerodynamic velocity directed towards the exposed surface. Therefore the powder particles tend to slow down their deposit velocity. This slowing has the effect that near the surface there is a dense layer of accumulated particles in balance between the various forces, viz electrostatic and aerodynamic, which prevents further deposit of the powder.

Figure 4 shows graphically what happens with the repulsion effects due to the electrical fields and particle charges. The deposited particles (12) represent a surface charge with associated electrical field (13) represented by the arrows which repel the new powder particles (14) represented as thickened with respect to the particles (15) emitted near the gun since they are slowed by the field (13). It is clear that as the air and powder mixture emitted becomes denser this phenomenon is accentuated.

Let us represent the strip velocity with the symbol Vn and that of the emitted powder with Vp. The optimal projection velocity of a necessary volume of powder to cover a given surface to prevent some of the above mentioned phenomena is equal to or less than Vn since in this case the powder emitted would always be deposited on a surface not having any.

To increase production we would have to increase the running velocity of the strip Vn to increase the surface exposed in the time unit which involves in turn that of projection Vp of the volume of powder triggering that chain of undesired phenomena mentioned above.

The system invented allows diminishing the effect of the gun electrical fields, cancelling the effects due to powder emission velocity and hence cancellation of the erosion effects and allowing greater production velocity. With reference to figure 5, the invention consists of a tension means or roller (22) placed in any position between the rollers normally used (2) and (3) so as to form a more or less acute angle and the two suction means or hoods (16) and (17) positioned so as to suck air only in parallel and not laterally with respect to the two new running planes of the strip (19) and (18). The paint guns are positioned at the height of the corner (20) formed by the two planes (19) and (18). From the laws of physics we know that electrical fields tend to be concentrated on the projecting parts of objects like tips or corners so that the tendency of the gun electrical field lines of force (21) in broken lines as in figure 6 concentrate on the corners (20) formed by the tension roller (15) with the strip (1) leaving the planes (19) and (18) practically without lines of force. By feeding the guns with powder the flow of the latter tends to divide in two with one part running parallel along the plane (19) and the other in the same manner along the plane (20). The running of the powder is helped by the suction hoods (16) and (17) which suck parallel to the plane (19) and (18) with laminar motion. This arrangement prevents the powder having an impact perpendicular to the strip surface with a_. reaction force such as to erode many particles from the surface.

As there is no suction in the rear part of the strip we can also avoid formation of turbulence on the strip edges which caused another type of erosion of the powder deposited thereon.

In this manner we can also overcome the problems caused by air and powder mixture density since in dividing the flow in two parts and sending it over two different broad surfaces, viz those of the planes (18) and (19), we decrease its density per exposed surface area unit to facilitate deposit. It appears evident that the powder particles are always deposited easily on the planes around the corners since the latter are without the electric repulsion field caused by the effect of the gun electrodes but only under the effect of the weaker electrical field generated by the electrical charge of the powder already deposited. Let us examine the powder and strip velocities along the lower plane (18) assuming the strip in motion in the direction of the arrow Fig. F Table 3 since the motion of the powder is parallel to that of the strip the relative velocity of motion is equal to the linear sum of the strip velocity Vn and powder velocity Vp. But the velocity of emission Vp of the powder along the plane (18) is increased by the velocity of the air sucked by the hood (17) independently of that emission and causing the further dilution required by the powder. The particles deposited along the plane (18) must have an electrical charge such as to allow adhesion to the surface and overcome both the suction force of the hood (17) and the force of gravity which tends to detach them. It is clear that the layer of powder deposited on this plane is made up only of particles having an excellent electrical charge with those having an insufficient charge to overcome the forces of suction and gravity being eliminated. This type of coating made up of particles with strong adhesion ensures a surface with excellent spreading and visual finish once it is polymerized in the baking oven as known .

On the upper plane (19) the operation is analogous despite the fact that the powder emitted encounters a surface already painted because of the lack of electrical field from the guns .

As on the plane (19) the force of gravity tends to cause deposit of the powder on the strip, we could also have the deposit of uncharged particles (phenomenon called powder- ing), the suction hood (16) is shifted into a position closer to the corner (20) and this is done in such a manner that its suction velocity is greater than that of the hood (17) so as to make up for the force of gravity effect which would oblige the uncharged particles to fall back on the strip.

The powder emission is divided mechanically along the two running planes. As imaginable for the technician, deflectors can be provided. Known shifting means in painting system (4) enable distri- bution of the painting powder on the two planes in a chosen manner by shifting the painting system upward or downward relative to the corner.

Basically the present invention enables: - elimination of the electrical field effect of the guns, dilution of the density of the powder emitted because of its distribution over a greater surface area and because of the suction which increases its velocity along the deposit surfaces independently of the projection density, elimination of the counter corona effects caused by the electrical fields, and increase of production speed thanks to the fact that increased powder dilution does not allow slowing effects as described of deposit near the surfaces. The present invention is described in a particular chosen application but possible variants are evident while remaining within the scope of the present invention. For example, tension means of other geometrical forms can be used instead of the strip tension roller (22).

It is also observed that the device in accordance with the present invention is easy and economical to construct despite the considerable advantages which it brings to the industrial process of electrostatic painting of metallic strips.

Claims

1. Device for painting with electrostatic powder of metallic strips, comprising a painting system which emits the powder toward the strip, characterized by strip tension means positioned in such a manner as to form around it a more or less acute angle with the strip; two suction means located along strip planes coming out of said angle, the suction means having suction parallel to said planes and the painting system emitting perpendicular to said angle formed by said planes .

2. Device in accordance with claim 1 characterized by a tension means comprising a roller.

3. Device in accordance with claim 1 characterized by the suction means comprising a suction hood.

4. Device in accordance with claim 3 characterized by air suction by the two hoods parallel to the two planes in which the strip is divided around the tension means.

5. Device in accordance with claim 1 characterized in that the two suction means have different suction flow.

6. Device in accordance with claim 1 characterized in that one suction means is located at a smaller distance than the other from the corner formed by said angle.

7. Device in accordance with claim 1 characterized in that one of said strip planes is over the other strip plane.

8. Device in accordance with claim 7 characterized in that the suction means corresponding the said one strip plane has suction velocity greater than the suction velocity of suction means corresponding the other strip plane.

9. Device in accordance with claim 7 characterized in that the suction means corresponding the said one strip plane is located at a smaller distance than the other from the corner formed by said angle.

10. Device in accordance with claim 1 characterized by the positioning of the painting system perpendicular to corner formed by said angle.

11. Device in accordance with claim 1 characterized in that the powder emission is divided mechanically along the two running planes.

12. Device in accordance with claim 1 characterized in that it enables dilution of the powder emitted by means of an external suction system.

13. Device in accordance with claim 1 characterized by dilution of the powder dividing its flow along two independent surfaces .

14. Device in accordance with claim 1 characterized by decreasing the electrical fields by causing them to close on a salient part such as the corner formed by the said angle.

15. Device in accordance with the claim 1 characterized by enablement of distribution of the painting powder on the two planes in a chosen manner by shifting means which shift the painting system upward or downward relative to the corner.

16. Device in accordance with the claim 1 characterized in that the tension means forming a curve around itself for a metallic strip.

17. Device in accordance with claim 1 characterized by at least two suction hoods .