WO1991006376A1

WO1991006376A1 - Spray gun with corona and tubular electrodes

Info

Publication number: WO1991006376A1
Application number: PCT/AU1990/000533
Authority: WO
Inventors: Frederick David Haig
Original assignee: Frederick David Haig
Priority date: 1989-11-06
Filing date: 1990-11-05
Publication date: 1991-05-16
Also published as: PT95786A; IE903982A1; PT95786B

Abstract

An electrostatic spray gun comprises at least one external corona discharge electrode (11) positioned so as to charge a stream of particulate matter entrained in a gas flow. Where the stream of particulate matter leaves the gun, the gun comprises a metal tube (7) charged to the same voltage as the corona discharge electrode (11) and through which tube (7) the particulate matter must pass prior to passing the electrode (11). In a preferred embodiment (not shown here) the electrode takes the form of a deflector which turns the stream of particulate matter through about 90°. The spray guns of the invention may be used for the spraying of hygroscopic materials such as sodium diacetate.

Description

SPRAT GUN WITH COBCNA AND TUBULARI_I_EC_EOIES

This invention relates to spray guns for use in electrostatic spray processes and more particularly to such guns for use in spraying hygroscopic materials.

Electrostatic spraying is a technique which is widely used in the application of coating powders. Basically, a powder is entrained in a gas stream and is passed through an electrostatic field before being sprayed on to a target (usually metal and earthed). A variety of electrode arrangements and gun conf gurations have been used, but they all adhere to this same basic principle.

Recently, it has become desirable to spray materials which have not hitherto been applied by electrostatic spray. One such material is solid material capable of liberating acetic acid in the presence of moisture. An example of such a material is sodium diacetate, and its application to dry meat carcasses to preserve the meat is described in PCT Published Application WO 90/03118. This material is quite different to coating powders; it is hygroscopic and tends to stick together. Moreover, even when comminuted and entrained in a gas stream for discharge through an electrostatic spray gun, it has been found that the spray guns currently available not only tend to clog up with powder but also have poor efficiency in terms of quantity of powder sprayed per unit time.

It has now been found that a spray gun which has been constructed as hereinunder described is capable of giving excellent results with such powders. There is therefore provided, according to the present invention, an electrostatic spray gun which comprises at least one external corona discharge electrode positioned so as to charge a stream of particulate matter entrained in a gas flow, and, where the entrained particulate matter leaves the gun, a metal tube charged to the same voltage as the electrode and through which tube the entrained particulate matter flows prior to passing the electrode.

It has been found that the use of an electrostatic gun of this type for the spraying of materials such as sodium diacetate gives not only better coating efficiency (that is, higher weight of deposited powder per unit surface), but also better penetration into corners and crevices. It is especially useful for the application of materials such as sodium diacetate to meat in order to enhance its shelf-life. The invention therefore also provides a method of treating meat by applying to the surface thereof a solid material which liberates acetic acid in the presence of moisture, the application being carried out by electrostatic spraying using at least one spray gun as hereinabove defined. It w ll readily be understood by the skilled worker that not only can more than one spray gun be used but also that these can be completely automated. This is especially useful in the spraying of chickens wherein not only should the exterior of the body by sprayed but also the body cavity. Careful positioning of a number of guns on an automated line can easily achieve this.

The entraining of the powder to be discharged through a spray gun according to this invention can be achieved by any convenient means, but especially preferred means are the dispensing apparatus described in copending Australian patent applications PJ7231 and PK1810, both in the name of the present applicant. The invention therefore also provides an electrostatic powder spraying apparatus comprising at least one electrostatic spray gun of the type hereinabove described, which gun is fed with coating powder entrained in a gas stream from a dispensing apparatus of the type described in the two copending patent applications mentioned hereinabove. Provided that the spray gun has the essential features hereinabove described, it can have any suitable configuration. In a typical embodiment, there is mounted axially with respect to the path of the powder to be sprayed a conical deflector with the electrode positioned beyond this deflector. Both of these components are mounted, for example, on a metal shaft which protrudes from the spray gun and which is coaxial with the metal tube, it being joined to the metal tube by some suitable conductive element such that the charge borne by the tube will be transmitted to the electrode. In a further embodiment of the invention, the metal shaft is tubular and the electrode is positioned at the end of this tubular shaft, or the shaft itself is suitably formed such that it can act as an electrode. This particular embodiment permits material to pass through the shaft and has been found to be particularly useful in the coating of cavities and crevices.

While these embodiments work well for many purposes, they can sometimes lead to difficulties when very hygroscopic powders are being sprayed. The intrusion of a shaft into the powder path can lead to unwanted accumulation of powder and even blocking of the gun.

This can be overcome by using a further particular embodiment of the invention. In this embodiment, the conical deflector and all supporting appendages are dispensed with, leaving the tube through which the powder exits completely clear and unimpeded. Mounted externally on the gun is a deflector whose purpose is to turn the path of the powder through about 90°. The deflector may be of any suitable configuration or material. In one very simple embodiment, it may be a flat plate which intersects the axis of the tube at 45°. At least one electrode may be fitted in any suitable location. In the case of the plate mentioned hereinabove, it may be mounted on the edge of the plate, or in a preferred embodiment the plate itself may be suitably shaped so that it forms the electrode.

The invention will now be further described with reference to the drawings.

Figure 1 is a longitudinal section of the end of a barrel of a spray gun according to one embodiment of the invention.

Figure 2 is a longitudinal section of the end of a barrel of a spray gun according to another embodiment of the invention. Figure 3 is a transverse section of the embodiment of Figure 2 along the line AA¹.

Figure 4 is a longitudinal section of the end of a barrel of a spray gun which is a modification of the gun of Figure 1. In the embodiment of Figure 1, a spray gun barrel consists basically of two coaxial plastic tubes 1 and 2, the annular space between them being filled with plastic foam 3. This annular space is closed completely by an annular wall perpendicular to the axis of the tubes. A metal tube 4 which is a coaxial continuation of tube 1 and of equal internal diameter extends beyond this closed end. This metal tube is externally threaded and is connected by wire 5 to a source of high electrostatic voltage. To this tube 4 is attached a gun head, generally designated as 6. The gun head comprises a metal tube 7 which is both internally and externally threaded, the internal diameter and threading being such that the tube 7 can be screwed on to the external thread of tube 4. Screwed on to the external screw thread of the tube 7 is a plastic guard 8. This shrouds the tube completely and defines at the end of tube 7 remote from tube 4 an expanding f usto-conical spray nozzle.

A planar metal strip 9 is rigidly attached diametrically across tube 7 such that electricity can be conducted between them (in this case, diametrically opposed slots in the tube have allowed the positioning of the strip, and this was followed by welding in place). The strip supports an electrode assembly such that the assembly is coaxial with the tube 7, the assembly consisting of a cylindrical metal rod 10 which protrudes beyond the frusto-conical spray nozzle. This rod is internally threaded at the end remote from the supporting strip and into this thread is screwed an electrode 11. An essentially conical plastic deflector 12 slides on to the rod 10. In practice, the electrode 11 is charged via wire 5, tubes 4 and 7, strip 9 and rod 10 to a high voltage. All of the components thus bear the same charge. The object to be sprayed is -earthed and an electrostatic field is established between electrode 11 and object. Particles of coating material entrained in a gas stream are carried along tube 1, through tubes 4 and 7 and out of the spray nozzle of plastic guard 8, the deflector 10 helping to disperse the particles. A feature which may usefully be incorporated when a substance such as sodium diacetate is to be sprayed is a conical deflector 13 which is situated at that end of rod 10 remote from electrode 11. This can be provided simply by shaping the end of the rod 10 such that it is conical. This deflector deflects material towards the inner surface of tube 7 and helps keep it clear of adhering material.

The embodiment depicted in Figures 2 and 3 shows basically the same spray gun barrel as depicted in Figure 1 except that the elements mounted coaxial ly with respect to the tube 7 have been removed and the tube presents a continuous bore from its function with tube 4 to its end remote therefrom. Attached externally to this tube by two metal rods 14 and 15 is a deflector plate 16. Where it joins the rods 14 and 15, this plate is parallel to the axis of the tube 7 but at a place remote from the tube 7, it is bent such that it intersects the axis at an angle of 45°. That part of the plate which intersects the axis is placed with respect to the end of tube 7 such that, and is dimensioned such that, all powder leaving the tube 7 strikes it. The end of the plate remote from the tube 7 converges to a point 17, and this acts as the electrode.

In use, this embodiment is pointed at right angles to the direction in which a conventional spray gun would be pointed. The plate deflects the powder spray, disperses it very effectively and charges it. Moreover the gun does not clog and the deflector plate remains remarkedly clean.

The embodiment of Figure 4 is nearly identical to that of Figure 1 and described hereinabove, with the exception that the assembly consisting of rod 10, electrode 11, deflector 12 and conical deflector 13 is replaced by an assembly which consists of an elongate tube 18, deflector 19 and electrode 20. Tube 18 is positioned where rod 10 was positioned in Figure 1. At the end of this tube remote from the strip 9 is a deflector 19 which takes the form of a hollow cone. In the mouth of the tube is fixed a flat pointed electrode 20.

The flow of material in this embodiment, when in operation, is similar to that in the Figure 1 embodiment, except that a portion of the material flows down the tube 18 past the electrode 20 on either side thereof and away from the gun. It has been found that this embodiment is especially useful for spraying into hollows and crevices.

Claims

The claims defining the invention are as follows:

1. An electrostatic spray gun which comprises at least one external corona discharge electrode positioned so as to charge a stream of particulate matter entrained in a gas flow and, where the entrained particulate matter leaves the gun, a metal tube charged to the same voltage as the electrode and through which tube the entrained particulate matter flows prior to passing the electrode.

2. An electrostatic spray gun according to claim

1, wherein the electrode is mounted on a metal shaft which extends coaxially from the metal tube to which is conductively connected.

3. An electrostatic spray gun according to claim

2, wherein the shaft is tubular.

4. An electrostatic spray gun according to claim 1, wherein the tube is clear of intrusions and there is fitted to the gun an external deflector which bears at least one electrode and which is adapted to deflect the path of the powder through about 90°.

5. An electrostatic spray gun according to claim 1 or claim 4, wherein the external deflector is a flat plate.

6. An electrostatic spray gun according to claim 1, 4 or 5, wherein the external deflector itself forms the electrode.

7. A method of treating meat by applying to the surface thereof a solid material which liberates acetic acid in the presence of moisture, the applicating being carried out by electrostatic spraying using at least one spray gun according to any one of claims 1-6.

8. A method of treating meat according to claim 7, wherein the solid material is sodium diacetate.

9. An electrostatic powder spraying apparatus comprising an electrostatic spray gun according to any one of claims 1-9 in combination with a metering apparatus according to Australian patent application P 1810.

10. An electrostatic powder spraying apparatus comprising an electrostatic spray, gun according to claim 2 in combination with a metering apparatus according to Australian patent application PJ 7231.

11. An electrostatic spray gun substantially as described with reference to the drawings.