Method and Apparatus for Re-Ionizing Insulating Powder in an Installation for Electrostatically Depositing Powder on Objects

The invention relates to a method and apparatus for re-ionizing an insulating powder in an installation for electrostatically depositing powder on objects. Here, re-ionization means electrostatically charging powder for a second time after it has previously been electrostatically charged. This is done e.g. by electrostatic spray guns.

Powder of high resistivity is electrostatically applied onto a part to be treated by spraying the part with a stream of powder electrified by charges produced by pins connected to a high-tension DC generator.

The electrified powder adheres to the part under the effect of electrostatic forces. Subsequent heat treatment applied to the part makes it possible to transform the layer of powder into a finished coat of paint or vitrified enamel.

U.S. patent application Ser. No. 97,112, filed Nov. 20, 1979, now U.S. Pat. No. 4,304,764 describes a booth which includes electrodes brought to a high electric potential, said electrodes being disposed at the inlet and at the outlet of an electrostatic powder-deposition chamber, but this booth does not make it possible to obtain good results. Not all the powder is charged or the powder is insufficiently charged. As a result, a more or less large fraction of the powder is not deposited on the parts.

In a booth in accordance with French Pat. No. 2 442 080 powder not deposited on a part either falls directly through a sifter into a recovery tank situated at the base of the booth or else is directed towards filters disposed in inlet and outlet modules.

The present invention aims to increase the proportion of powder which is deposited on the parts to be treated.

The invention provides apparatus for re-ionizing insulating powder in an installation for electrostatically depositing powder on objects passing therethrough, said installation including a powder-spraying chamber and air-filtering units for sucking in powder that is not deposited on the objects, wherein the re-ionizing apparatus comprises at least one re-ionization unit to re-ionize the non-deposited powder, the re-ionization unit being situated in a re-ionization chamber which is adjacent to the powder-spraying chamber, the re-ionization chamber having walls extending transversely to the direction of motion of the objects through the apparatus to direct the powder towards said objects on which powder is to be deposited, said walls being situated between the powder-spraying chamber and the suction openings via which air is sucked through the filtering units.

The invention also provides a method of re-ionizing powder in an installation for electrostatically depositing powder on objects, said installation including a powder-spraying chamber, wherein the method consists in using powder-re-ionization means disposed at the outlet of the powder-spraying chamber in a powder-re-ionization chamber and in placing walls transverse to the direction of motion of the objects along the path of the powder to direct the powder towards the objects to be treated.

A preferred embodiment of the invention is described hereinbelow with reference to the accompanying drawings in which:

FIG. 1 is an elevation of a powder-deposition booth which includes re-ionization units in accordance with the invention; FIG. 2 is a plan view of the booth of FIG. 1; and

FIG. 3 is a cross-section of a re-ionization pipe.

In the embodiment illustrated, parts 1 placed on a conveyor 2 pass through a powder-depositing booth which includes three modules A,B,C: module A serves as an inlet air-lock, module B is the spray chamber where powder is to be deposited on the parts by means of projection units 22, and module C is an outlet air-lock. The inlet module A and the outlet module C contain vertical cylindrical filters 5 which are, for example, four in number. A fan 8 placed in a box 12 sucks air in through the filters, and an unclogging unit (not shown) is used to regenerate the filters.

A fluidizing unit 3 situated at the bases of the filters recovers the powder which falls from the filters and returns it to a powder store 11 situated in module B of the booth. Module B contains a fluidization unit 9 and a sifter 20. The powder depositing units 22 can be of any type, e.g. electrostatic spray guns. The powder depositing units 22 are connected to injectors 28 which draw off the powder directly by means of dipping tubes 10 in the powder store 11.

The inlet module A and the outlet module C have respective re-ionization compartments D and E, adjacent to the central module B. These re-ionization compartments could alternatively be disposed in the central module B. A re-ionization compartment is constituted by solid walls 13,14,15,16 which leave a central passage 17 for the parts to be treated. These walls constitute obstacles in the path of the powder and thereby help to confine the powder in the powder deposition module B, causing considerable loss of head when the powder passes from the powder deposition chamber B to the filtering chambers A and C. These obstacles oblige the powder which takes part in this transfer to move towards the central axis 18 of the booth in the direction of arrows f.sub.1, f.sub.2 and thus towards the parts which move through the booth. The fraction of powder which is still charged is redeposited on the parts. The re-ionization compartments D, E further include ionization units 21, 23, 25 and 27 constituted by insulating tubes extending vertically up the entire height of the booth. Pins 29 pass through holes 30 in these tubes, leaving a space around the pins where they pass through the holes. The pins are fixed on the tube along the generatrix that is furthest from the holes by a fixing means such as nailing or the like. The pins 29 point towards the centre of the booth perpendicularly to the path followed by the parts, and pass through an electrically conductive member 31 which is connected to the high-tension supply by a conductor 32. The high-tension supply is an electrostatic generator which can be the same as that used for the spray guns in module B; the pins 29 are thus in contact with the conductive member 31 which brings them to high potential for re-ionizing the powder. The conductive member 31 is constituted by a helical spring suspended along the axis of the tube with the pins 29 passing between its turns. The conductive member 31 could alternatively be constituted by metal braiding. The pins are disposed along a generatrix along the entire height of the tube; the pins should be spaced at a spacing lying between 10 mm and 100 mm apart, for example.

To avoid electrostatic leakage, the insulating tube 21 is closed at one end by an insulating plug 33 through which the high-tension input conductor 32 passes; the other end of said insulating tube is connected to a compressed air supply unit (not shown) via a tube 34.

During operation, the pins 29 are brought to a high electric potential by means of the member 31 so as to ionize the powder, the tube being supplied with compressed air which, on leaving via the holes surrounding the pins, prevents powder from being deposited on said pins, as this would impair proper ionization of the air and of the powder in the compartment.

Use of this system not only increases the powder deposition efficiency by making the powder move towards the parts and by re-ionizing the powder, but also, by increasing direct recovery in the powder deposition chamber and by improving the efficiency of deposition, reduces the quantity of powder deposited on the filtering components thereby facilitating their unclogging. The walls of the re-ionization chamber increase the density of powder in the powder deposition chamber.

Further, the system improves the evenness of the thickness of the powder deposition on the parts. Suction by the filters evens out the density of powder up the entire height of the booth and therefore provides uniform deposition on the parts.

A plurality of successive re-ionization chambers can be disposed along the path of the powder as it goes from the powder-spraying chamber to the filters.