The invention is concerned with electrostatic discharge nozzles of the type used (for example) in the application of coatings to sheet metals in rolling mills and the like.

The electrostatic application of coatings to sheet metal in a rolling mill is well known. Thus, the metal is afforded a degree of corrosion protection and, or lubrication which is useful during further processing of the material such as pressing, canning etc. In order to provide a reliable, predictable degree of protection, a homogeneous and even distribution (uniform thickness) of coating material (for example oil) is desireable.

The coating is typically deposited using an applicator (spray nozzle) having an annular spray gap. The present invention is concerned with improvements to such applicators.

In modern rolling mills, the high throughput of metal places heavy demands on the coating applicator systems. For example, Temper and Skin pass mills can typically pass metal with speeds in excess of 1500m/minute with coating thicknesses of up to 3.5g/m ² .

These demands frequently exceed the capacities of typical coating applicators. For example, a typical applicator might be able to deliver up to15cc of coating material per cm of applicator spray length but with a line speed of 1000m/min, and a coating thickness of 2g/ m ² , the applicator is required to deliver 22cc material per cm of strip.

This causes 'saturation' of the applicator edge leading to large or heavy droplets (coarse atomisation) of the material which in turn leads to uneven coating of the strip and/or application of a non-homogeneous layer of coating material.

One know approach to solving this problem is to increase the number of applicators arranged to deposit material on the strip but this is constrained by space limitations. Another approach involves the application of an electrostatic charge to the applicator. Such a charge facilitates atomisation of the oil or other coating material as is exits the applicator to form a fine mist. Again, however, there are practical limits on this solution.

It is known that for a given electric charge imparted to an object of given mass, the most intense electric field is realised where the object tapers to a point. For example, for a charge carrying object having a typical teardrop shape with one end being substantially hemispherical and the other tapering to a point, the intensity of the electric field is greatest around the point.

It is an object of the invention to provide a solution to the problems outlined above.

According to the invention a nozzle for applying material comprises:

an inner cylindrical portion arranged concentrically with an outer cylindrical portion to define an annuar gap therebetween, and is characterized by at least one of the portions having a radial thickness decreasing along its axial length to a minimum value proximal to an end of the device where material exits.

In one embodiment, the inner portion has a decreasing radial thickness.

In a preferred embodiment, the nozzle is arranged to apply material to sheet metal in a rolling mill. The nozzle may be used to apply oil, for example kerosene, to the sheet metal.

According to a second aspect of the invention, a method of applying material to a substrate comprises:

providing a nozzle having an inner cylindrical portion arranged concentrically with an outer cylindrical portion to define an annuar gap therebetween, wherein at least one of the portions has a radial thickness decreasing along its axial length to a minimum value proximal to an end of the device where material exits and

arranging the nozzle such that material exiting therefrom is directed to the substrate and

directing a fluid comprising the material to the nozzle.

In one embodiment, the inner portion of the nozzle has a decreasing radial thickness.

In a preferred embodiment, the method comprises applying material to sheet metal in a rolling mill. The material could be oil, for example kerosene.

The invention is described herein, by non-limiting example, with reference to the appended figures in which:

figure 1 illustrates the distribution of electric charge in an object having a fine point;

figure 2 illustrates a spray nozzle according to the prior art;

figure 3 illustrates a typical spray pattern obtained using a nozzle according to the prior art;

figure 4 illustrates a spray nozzle according to an embodiment of the invention;

figure 5 illustrates a typical spray pattern obtained using a nozzle according to the invention and figure 6 illustrates a spray nozzle according to an alternative embodiment of the invention.

The figures are schematic only and are not drawn to scale. Moreover, certain dimensions, gaps or angles may be exaggerated for illustrative purposes.

Referring to figure 1 , it is known that where a given charge is imparted to an object having a shape which tapers to a point 1 , the observed electric field is most intense in the region around that point. Without being bound by theory, it is believe that this phenomenon arises because unbalanced repulsion between charge carriers (e.g. electrons) in the region of the point and charge carriers in the body of the object gives rise to a higher concentration of charge carriers around the point.

Referring to figure 2, an electrostatic spray nozzle according to the prior art includes a solid cylindrical inner portion 2, arranged concentrically with a hollow cylindrical outer portion 3. The outer diameter of portion 2 and the inner diameter of portion 3 are selected to define an annular gap 4. Coating material such as oil is directed to the annular gap via ducts (not shown) and exits as shown at 5.

Portions 2 and 3 are formed of electrically conducting material such as metal and. there is electrical continuity between the two, the potential being applied between both and ground.

Referring to figure 3, an electrical potential is applied to the prior art device giving rise to an improved spray pattern, whereby the material gathers at substantially evenly spaced cusps 6 around the annular gap and exits the device as a number of streams 7 of atomised material.

Referring to figure 4, in a particular embodiment of the invention, the inner portion 2 of the applicator has a hollowed region at the end of the device where coating material exits, the hollowed region being shaped such that the inner portion exhibits a progressively decreasing radial thickness from a value (say R2) distal of the annular gap to a minimum value (Rmin) in the region of the annular gap.

By this arrangement, the invention provides a three dimensional (and in this case circular) analogue of the field concentrating point illustrated in figure 2 and the effect of an electrical potential of given value on the spray pattern is enhanced.

Referring to figure 5, the inventors have observed an improved spray pattern when employing an applicator according to the invention whereby the number of cusps 6 and streams 7 are increased, the streams are finer and the spray pattern is narrower. The narrower spray pattern allows for a greater number of applicators per unit area of sheet material, thus further assisting in meeting the coating material flow requirements.

Figure 6 illustrates one possible alternative shape for the inner portion 2. This is another of a large number of possibilities, the essential feature being that the radial thickness of at least one of the portions decreases along the axial length of the device toward the end where material exits. In other embodiments, this feature could be exhibited by the outer portion.