GB2204254A

GB2204254A - Electrostatic dry-powder spraying

Info

Publication number: GB2204254A
Application number: GB08706916A
Authority: GB
Inventors: Stanley Macdonald
Original assignee: AUTO PAINT SYSTEMS Ltd
Current assignee: AUTO PAINT SYSTEMS Ltd
Priority date: 1987-03-24
Filing date: 1987-03-24
Publication date: 1988-11-09
Also published as: GB8706916D0

Abstract

In electrostatic dry-powder spray booths wherein air is drawn through the booth from an open face past the object to be coated and through a filter system, one or more low-potential plates are arranged between the object being coated and the filter system. Dry-powder which does not coat the object collects preferentially on the plates from which it may subsequently be recovered for re-use. The plates are at a potential lower than the coating material being sprayed. Earth potential is preferred. The plates may be solid, perforated, chain-linked, woven or non-woven, or in the form of a screen. A second plate in the form of a membrane may be employed next to the face of the filter.

Description

ELECTROSTATIC DRY-PDER SPRAYING This invention relates to electrostatic dry-powder spray booths, especially, non-automated booths, and the incorporation therein of a highly efficient poder-recovery system.

In the electrostatic spraying of dry-powder coatings the object to be coated is suspended in a ventilated enclosure which has an exhaust and filter system integral therewith or connected thereto. The positively charged coating material, being charged either electrostatically or by means of an electric generator, is projected towards the object to be coated typically by means of a gun, or similar equipment, powered by compressed air. The object to be coated is usually at earth potential and attracts the positively charged powder to its surfaces, thus becoming coated. An air exhaust fan is employed to create a moving air stream at the face of the spray booth, through the coating chamber and thence through the filter arrangement.One of the disadvantages of the present practice is that as only approximately 20t of the coating material is deposited upon the object to be coated, there is a large amount of overspray which has either to be collected for re-use for economic operation or is lost, at significant economic loss to the operator. The overspray is either deposited on the booth casing, falls to a lower hopper situated at the base of the spray booth or other convenient place, or is deposited on and collected by filter elements. When the elements are completely full, the full elements are discarded and replaced by fresh units.

The material collected by the filter represents approximately 40% of the materials sprayed, and as materials commonly being sprayed cost in the order of 3 4 per kilogram, that which is collected on the filter medium represents a significant economic loss to the applicator. It is clear therefore that recovery of the overspray, i.e. the unused coating material, in a re-usable condition is of important economic value to the applicator.

The present invention provides a low cost way of significantly increasing the recovery of overspray in an electrostatic dry-powder spray booth. The invention comprises the placement between the object being coated and the filter medium of the spray booth, of one o more "low-potential" plates. By "low-potential" plates is meant any plate(s) or membrane or other form of screen which is at a lower electrical potential then the coating material being sprayed. Preferably, for ease of installation, the low potential plate(s) are at earth potential.

The area of the low potential plate(s) is preferably, but not necessarily, greater than the total face area of the filter and may mask the filter. The plate(s) may be of planar, convoluted planar, curved of convoluted curved shape or configuration and may be corrugated, dimpled or otherwise formed to increase its effective surface area. The low potential plate(s) may be solid, perforated or pierced, chain linked, woven or non-woven, or take the form of a fabricated screen and may be of metal or other material which is electrically conductive and/or electrostatically chargeable.

In practice the electrostatically charged coating material is projected towards the object to be coated, and when using my invention the majority of the overspray is deposited on the interposed low potential plate(s) or membrane both on the forward facing and reverse surfaces by reason of the electrical potential difference between the plate and the coating material being sprayed. The coating material deposited on the low potential plate(s) can be easily removed therefrom and recovered for re-use by brushing, shaking, rapping or otherwise.

In certain circumstances it may be desirable to use a secondary "low-potential" plate in the form of a mmbrane of electrostatically chargeable material placed on or closely adjacent to the face of the filter. This secondary "low-potential" plate or membrane, which in certain situations may be used without the use of a primary "low-potential" plate, brings about a significant improvement in the efficiency of the filter and greatly assists in the recovery of overspray and the prolonging of the life of the filter element.

The advantages of my invention are that a significantly greater percentage of the powder being sprayed is recovered for re-use, resulting in lower material costs. Filter life is prolonged considerably as the material loadings in the filter medium are significantly reduced d per unit of time in use and one also obtains lower labour costs from easier and quicker cleaning down procedures when changing the colour or specification of powder being sprayed.

Claims

1. An electrostatic dry-powder spray booth including a coating chamber in which an object to be coated may be suspended, a filter and an air exhaust fan arranged to create a moving airstream passing from a face of the spray booth, through the coating chamber and then through the filter, the booth including one or more low-potential plates located between the object being coated and the filter.

2. A spray booth according to claim 1 wherein the low-potential plate is arranged to be at earth-potential in use.

3. A spray booth according to claim 1 or 2 wherein the area of the low-potential plate(s) is greater than the total face area of the filter.

4. A spray booth according to any one of claims 1 to 3 wherein the low-potential plate(s) is arranged to mask the filter.

5. A spray booth according to any of claims 1 to 4 wherein the low-potential plate(s) is corrugated, dimpled or otherwise formed to increase its effective surface area.

6. A spray booth according to any one of the preceding claims and including at least two low-potential plates, one of which is in the form of a membrane of electrostatically chargeable material placed on or closely adjacent to the face of the filter.

7. An electrostatic dry-powder spray booth according to claim 1 and substantially as hereinbefore described.