Apparatus For Covering An Object With A Layer Of Powder

Abstract

This invention concerns a spray nozzle fed with the powder conveyed in an air stream constructed so that expansion of the air conveying the powder is effected in the nozzle and the spread of the spray jet issuing from the nozzle is adjusted by regulating the flow of an auxiliary jet of air which whirls about the air stream conveying the powder. In a specific embodiment the nozzle comprises a conduit having a downstream portion of larger cross-section than the cross-section of an upstream portion thereby forming an expansion chamber in the nozzle; and the jet of whirling air is admitted to the nozzle conduit downstream of the expansion chamber. A conductive core is mounted coaxially in the nozzle conduit and is adapted to be connected to a source of high voltage. The ionization means for ionizing the air conveyed powder are at the outlet of the nozzle.

Description

The present invention relates to a method of and apparatus for covering an object with a layer of powder wherein a cloud of powder is directed towards the said object to be coated across a zone of high ionization created by an electric member carried to a high potential with respect of that of the object to be coated. In a particular embodiment of this method, a secondary jet of air of a whirling nature is caused about the axial jet of air conveying the powder, which allows this latter to spread considerably in a direction transverse to the direction towards the object, therefore allowing a more uniform and rapid covering of an object to be obtained in numerous applications.

However this type of method is limited in use for certain objects having particular shapes and more particularly, in the case of objects having hollow shapes in the center of convex shapes it presents difficulties in coating these two very different shapes. In fact, if one is content to use a spray gun having a direct axial jet, the cloud of powder may be moved so as to be deposited in the hollows, but on the contrary a very limited coating is obtained on account of the surface. In such cases one is usually obliged to make do with touching-up with the aid of different spray guns, by using firstly a spray gun with a large opening and then a directive spray gun having a small opening, or vice versa.

The present invention has for an object to provide a method and apparatus which allows this drawback to be reduced or avoided.

The method according to this invention effects, in the spray nozzle, an expansion of the current of air conveying the powder and the spread of the sprayed jet is adjusted to be larger or smaller by adjusting the flow of an auxiliary jet of air coaxially whirling about the air stream conveying the powder between a flow resulting in the formation of a direct jet and a higher flow resulting in a wide spread of the spray. Using these methods allows one direct jet to be provided at a relatively low axial speed on account of the expansion which has taken place, while the sprayed jet may easily be adapted at any time, and while operating, to the surface form during a coating.

The invention also provides apparatus for carrying out the method, comprising a nozzle traversed by a conduit for a gaseous stream conveying the powder, ionization means at the outlet from the said nozzle, means for forming a jet of air coaxially whirling about the air stream conveying the powder and wherein the nozzle conduit has a downstream portion of a substantially larger cross section than that of an upstream portion so as to form an expansion chamber.

The invention will now be further described, by way of example, with reference to the accompanying drawing, which is a schematic sectional view of one embodiment of atomization nozzle according to the invention.

Referring to the drawing, an atomization nozzle comprises a feed tube 1 for a current of air conveying powder from a powder source (not shown) to the nozzle. The nozzle itself has an upstream nozzle conduit part 2 made of a conductive material, followed by a downstream nozzle conduit part 3 made of an insulating material, and an inner core 4 arranged so as to define an annular channel between these two members 3 and 4. The core 4 is conductive and widens into a radial edge 5 forming an electrode and is terminated at its downstream end by an insulating domed head member 6, the maximum diameter of which is less than that of the diameter of the edge 5. The core 4 is supported from the part 2 by arms 7.

The nozzle conduit part 3 terminates at a rounded outer edge 8 which flares outwardly and incorporates perforations or passages 9 discharging into a zone situated at the beginning of the edge 8. These passages 9 communicate with an outer annular conduit 10 formed between the outer face of the nozzle part 3 and an elongated threaded nut 11 securing the nozzle on the spray gun body 14. In this embodiment the passages 9 are inclined in transverse planes with respect to the radial directions so as to abut in a substantially tangential way the inner surface of the nozzle conduit part 3. They are also, as can be seen from the drawing, slightly inclined forwardly in the axial direction, for example at 15.degree..

The body of the spray gun 14 is an insulating hollow barrel through which passes the powder feed tube 1 and the high voltage cable 15, while the interstitial space in the barrel 14 is used to conduct the air which is fed to the passages 9.

The nozzle conduit has a cross section increasing substantially between the inlet end 15 defined by the cylindrical tip 2a of the conduit part 2 and the annular conduit 13 situated between the conduit part 3 and the core 4. This increase in cross section occurs in a progressive manner by providing the conduit part 2 with a conically diverging portion 2b. Advantageously the core 4 has an upstream pointed part 4a which assists the flow of air conveying the powder and reduces any obstruction of this latter.

In operation, the air conveying the powder is fed in through the conduit 1 and this air expands largely in the annular chamber 13 between the core 4 and the conduit part 3. This expansion causes a reduction in speed in the axial direction which favors a particularly suitable electrostatic directive spray, that is to say without too much axial speed. The current of air whirling about the axial stream of air induces a spreading of the air jet conveying the powder which allows the covering of large areas. This spreading of the air stream conveying the powder may be adjusted while operating, according to the form and design of the object to be coated. This can be achieved simply by varying the flow of air introduced into the annular conduit 10, for example by means of a valve. It results that the shape of the powder spray may be adjusted from a practically cylindrical form, such as is shown at 12 at the outlet of the nozzle conduit 3, to a very pronounced spread which corresponds substantially to the cone formed by the tangent to the area 8 a little upstream of its free end. By reason of this arrangement optimum condition can be obtained in the coating of bodies of very varying shapes and more particularly of large shape having hollow surfaces. It is sufficient for the operator to act on the air adjusting valve in the conduit 10 to ensure a larger or smaller spreading of the air jet.

It will be understood that the invention is not limited to the embodiment described and shown. Thus the nozzle may be produced entirely or partly of a conductive material.

Claims

I claim:

1. Apparatus for coating an object with a layer of powder, said apparatus comprising a nozzle defining a conduit for a gaseous stream conveying the powder, the downstream portion of said nozzle conduit being substantially larger cross section than its upstream portion, a conductive core mounted coaxially in said nozzle conduit and adapted to be connected to a source of high voltage, ionization means at the outlet of said nozzle, and means for forming an adjustable spray of air coaxially whirling about the gaseous stream conveying the powder, in which said spray-forming means discharge into said downstream portion of said nozzle conduit.

2. Apparatus according to claim 1, wherein the nozzle has an outlet portion defining a terminal part of the nozzle conduit which outlet portion widens outwardly to a rounded edge, and the means for forming a spray of coaxially whirling air comprises a plurality of passages in said nozzle discharging into said conduit upstream of said terminal part.

3. Apparatus according to claim 2, wherein the passages are all inclined to intersect the conduit substantially tangentially.

4. Apparatus according to claim 3, wherein the passages are slightly inclined forwardly.

5. Apparatus as claimed in claim 1 in which the portion of said nozzle defining the downstream portion of said conduit is made of an electrical insulating material, and the portion of said nozzle defining the upstream portion of said conduit is made of an electrically conductive material.