THIS INVENTION relates to a powder charging system incorporated in an applicator for applying electrostatically charged powdered material to a surface to be coated. The charged particles are attracted to and adhere to the surface which is earthed or subjected to an electrostatic charge of opposite polarity. The coated surface is then subjected to heat to fuse the particles and bond the resultant film to the surface. Such applicators (hereinafter referred to as "applicators of the type described") generally comprise means located adjacent the inlet end of the applicator for entraining the powder in a stream of air or other gas, a charging duct formed of a electrically non-conducting material through which the entrained material passes so that frictional contact generates an electrostatic charge on the particles of powder, and a discharge nozzle for directing the charged particles toward the surface to be coated. Any uncharged powder particles and any charged particles which do not reach the surface fall to the floor and may be recovered for subsequent use.

PRIOR ART

The efficiency of the applicator is governed by the effectiveness of the powder charging system. The greater the proportion of particles which can be charged the less material is required to be recycled. Generally it is considered that in its passage through the charging duct the entrained material should be subjected to considerable turbulence to ensure maximum contact between the particles and the walls of the duct. One applicator which has been widely used is described in Australian Patent Specification Nos. 543354 and 543360. The charging portion of the applicator comprises a number of elongated curved charging

ducts made of an electrically conducting material. In the event that the duct deteriorates with extended use it is necessary to replace the entire charging duct. Various other configurations of charging duct are disclosed in International Publication Nos. WO 82/03573 and WO 88/08332.

THE INVENTION

It has been found that if the charging duct is in the form of an annular passage, the diameter of which increases to a maximum and then decreases progressively to a minimum a number of times along the length of the duct there is an increase in the charging efficiency of the applicator, and further it has been found that it is possible to construct the duct from a number of identical components so that in the event of deterioration in the efficiency of the duct it is possible to dismantle it and replace the one or more defective components.

Thus in one form the invention resides in an applicator of the type described characterised in that the charging duct is a substantially annular passage, the diameter of the annular passage increasing to a maximum and then decreasing to a minimum progressively a number of times along the length of passage.

In another form the invention resides in a applicator of the type described characterised in that the charging duct comprises a plurality of components formed of an electrically non-conducting material assembled in end to end relationship along a common axis, said components being so shaped or contoured that in the assembled position they provide one or more non-linear tortuous passages through which the powdered material passes.

DESCRIPTION OF EMBODIMENTS

The invention will be better understood by reference to the following description of the embodiment shown in the accompanying drawings wherein:-

Figure 1 is an elevation showing the general outline of the applicator;

Figure 2a is a sectional view of the inlet end of one embodiment of the applicator;

Figure 2b, which is a continuation of Figure 2a, is a sectional view of the discharge end of the applicator;

Figure 3 is a fragmentary sectional view showing a second embodiment of the invention;

Figure 4 is a fragmentary sectional view of a third embodiment of the invention; and

Figure 5 is a fragmentary sectional view of a fourth embodiment of the invention.

As shown in Figures 1, 2a and 2b of the drawings the applicator housing 11 is generally cylindrical in form having means 12 at the inlet end for coupling it to a hose (not shown) through which the powdered material is fed and an inlet 13 for connection to a supply of compressed air. A discharge nozzle 15 is fitted to the discharge end. A terminal 14 for earthing the applicator is also provided.

The first section of the applicator generally designated 16 in Figure 2a comprises means for entraining the powdered material in a stream of air i.e. fluidising the powder so that it will flow through the second section generally

designated 17 in Figures 2a and 2b before being discharged through the discharge nozzle 15. The construction of the first section 15 of the applicator is the subject of a concurrent patent application related to our Australia Provisional Application PK4168/91 and is fully described therein. Whilst it is preferred that the powdered material be entrained using the construction shown in section 16 any other suitable means for entraining or fluidising the powder may be used.

In accordance with the invention the charging duct comprises an outer tube 18 of aluminium or other electrically conducting material fitted with end caps 19 and 20 which are externally threaded to engage correspondingly threaded portions of the casing 11. The tube 18 is connected to the earthing terminal 14. A plurality of substantially cylindrical outer modules 23 are fitted into the' tube 18 in end to end relationship and located by end modules 24 bearing against the end caps 19 and 20. The axial bore through each of the outer modules 23 tapers inwardly from each end so that the bore has a larger diameter at each end than at the middle. The end modules 24 have a short section 25 of bore corresponding to the minimum internal diameter of the modules 18 and a section 26 of bore which tapers outwardly to the maximum internal bore of the modules 18. An assembly of bead like members 27 are mounted in end to end relationship on a rod 28 which extends between the end caps 19 and 20 and held in place by screws passing through the end caps. The bead like members are essentially in the form of a pair of frusto-conical portions with abutting bases. The bead like members are located so that they bridge a pair of adjacent outer modules 18 by providing end members 29 at each end of the rod. The end members are cylindrical and have an external diameter corresponding to the minimum external diameter of the bead like members 27. The outer annular

modules 18, the end modules 24, the bead like members 27 the end members 29, the end caps 19 and 20 and the rod 28 are all formed of an electrically non-conducting material. The preferred material is a tetra fluoro ethylene polymer such as that marketed under the trade mark "Teflon" . It may be reinforced with carbon particles if desired.

The passage defined by the outer modules and the bead like members is annular and its diameter increases from a minimum to a maximum and then returns to a minimum progressively several times along the length of the passage. The movement of the entrained powdered material through such passage ensures that there is sufficient turbulence to bring the particles of powder into frictional contact with the walls of the passage so as to generate an electrostatic charge on the particles.

In the embodiment of the invention shown in Figure 3 of the drawings the profile of the inner surface of the outer modules 31 and the profile of the outer surface of the bead like members 32 is such that the annular passage through which the powder particles pass to pick an electrostatic charge has sections 33 of increasing and decreasing diameter separated by short sections 34 of constant diameter.

In the embodiment shown in Figure 4 of the drawings the peak of the outer surface of the bead like members 41 is flattened to increase the degree of turbulence.

In the embodiment shown in Figure 5 of the drawings the charging duct is formed by an assembly of discs 51 each of which has a series of passages 52 passing from on face of the disc to the other, each passage forming part of a spiral. The discs are assembled so that the passages are

aligned to form a series of spiral passages through which the particles of powder pass to pick up an electrostatic charge.