Electrostatic blade coaters and methods of use

Abstract

 © Electrostatic coating apparatus and method, principally for conductive coating media, comprise a pair of applicators (4, 5) positioned on either side of the path (X-X) of movement of the workpiece through the apparatus and maintained at earth potential. A pair of charging members (6, 7) are also positioned on either side of the path (X-X) of movement of the workpiece and are spaced from the applicators (4,5) so as to charge the medium only after the medium has left the applicators (4, 5). When the path (X-X) of workpiece movement is horizontal, a booster charging member (8) is provided below that path to deflect the medium towards the workpiece. The booster (8) is supplied with a voltage of the same magnitude as that of the lower charging member (7). The polarity of the voltage supply to the booster (8) may be the same as or opposite to that of the lower charging member (7).

Description

[0001] This invention relates to electrostatic blade coaters and to methods of electrostatically coating an article with a liquid. The word "blade" as used herein is intended to have a wide meaning encompassing any suitable applicator provided with an edge from which the coating medium is discharged or drawn off.

[0002] A conventional electrostatic blade coater has an applicator blade which is supplied with a liquid and which incorporates an electrical conductor (also known as a charging electrode) for imparting to the liquid a high voltage charge. The charged liquid is drawn off the blade edge and divides to form a mist of charged droplets which are attracted to the surface of the article to be coated.

[0003] When this technique is used for liquids which are electrically conducting (such as some oils and paints and aqueous liquids) precautions must be taken to prevent the electrical charge from leaking or being conducted upstream through the liquid itself and thence through any electrically conducting parts of the liquid supply system. Hitherto, this problem has been met by electrically insulating the applicator blade from the upstream liquid supply system and incorporating insulators, known as voltage breaks, in the supply system. The present invention aims to solve this problem in a simpler manner, enabling electrostatic blade coaters to operate with electrically conductive liquids, such as aqueous paints or emulsions, or electrically conductive oils.

[0004] According to one aspect of the invention an electrostatic blade coater has an applicator and electrostatic charging means for drawing off the blade a liquid to be coated on an article, the blade being substantially at earth potential.

[0005] Since the applicator is at earth potential the liquid in and upstream of the applicator and the liquid supply system upstream of the applicator may also be at earth potential, so avoiding any need to insulate the applicator or to provide voltage breaks in the supply system. A coater in accordance with the invention is therefore simpler and less hazardous than known coaters in which the electrostatic charge is applied to the liquid as the latter passes through or mover the applicator.

[0006] The electrostatic charging means may be of any convenient construction, e.g. an array of thin wires or a thin wire and a conductive sheet. Preferably, however, the charging means include a charging blade spaced from the applicator, itself in the form of a blade and acting not only to draw the liquid from the applicator blade but also to charge the liquid so that the liquid forms a finely divided mist of droplets which are drawn to an adjacent surface of the article to be coated by electrostatic attraction.

[0007] The charging blade may be spaced from the applicator blade by a distance of the order of 20 cm or more: a smaller spacing tends to cause arcing between the charging blade, which in use is typically at a voltage of the order of 90 kV, and the earthed applicator blade.

[0008] The liquid may be fed e.g. from the interior of the applicator blade by gravity to the edge of the applicator blade, being evenly distributed along the complete length thereof before being drawn off the edge, the edge being in the form of an overflow, hereafter called a "weir". An alternative arrangement is for the liquid to issue from a series of spaced nozzles, such as hypodermic needles, of interchangeable size, the nozzles then replacing the blade edge or weir previously mentioned.

[0009] Normally, the article to be coated is passed along a generally straight line through the coater there being an applicator and a charging member on either side of the article. The direction of the path of movement may be horizontal or vertical, or inclined to the horizontal and vertical. However, when the said path is generally horizontal, the lower applicator and charging member are preferably supplemented by a further member, herein called a booster blade, positioned to influence the path of movement and spread or distribution of the liquid in its passage from the applicator to the article to be coated.

[0010] The polarity of the voltage supplied to the booster blade may be the same as or opposite to the polarity of the voltage supplied to the charging blade.

[0011] Each applicator (blade) is preferably longer than the width of the article to be coated to ensure that the marginal areas of the article are effectively coated with the liquid.

[0012] According to another aspect of the invention, a method of electrostaically coating an article with a liquid comprises applying an electrostatic charge to the liquid after the latter has been drawn off or discharged from an applicator and before the liquid has reached the article.

[0013] The electrostatic charge is preferably applied to the liquid by means of a charging blade, arranged substantially parallel to the applicator but spaced therefrom sufficiently to avoid arcing between the charging blade and the applicator.

[0014] An electrostatic blade coater according to the invention will now be described in greater detail, merely by way of example, with reference to the accompanying drawing which is a diagrammatic view of one preferred embodiment of an electrostatic blade coater.

[0015] In the illustrated embodiment, the coater applies a coating of an aqueous electrically conducting liquid to both sides of a flat, elongated article, such as steel strip, shown in broken lines.

[0016] The strip passes in this example through the coater along a generally horizontal path in the direction X -X. The liquid is supplied from a transfer pump 10 through tachogenerator-controlled metering pumps 11 and pipes 2 and 3 to upper and lower applicator blades 4 and 5.

[0017] Each blade 4 and 5 extends generally horizontally and has along its length a series of holes from which the liquid issues, so that the distribution of liquid is even along the length of each blade. Each blade 4 and 5 has a thin edge 4a or 5a forming a weir over which the liquid passes.

[0018] The liquid is attracted away from the edge 4a of the upper blade 4 and into an electrostatic field developed by a horizontal charging blade 6 at a negative potential of about 90 kV. The blade 6 is parallel to and spaced from the blade 4 by a distance of about 20cm, and the blade 6 is slightly below the blade 4. The liquid, finely divided into charged droplets forms a mist which is attacted to the upper surface of the steel strip which is at earth potential.

[0019] A similar, lower charging blade 7 is parallel to and spaced from the lower applicator blade 5. The blade 7 is also at a negative potential of about 90 kV and generates an electrostatic field into which the liquid is drawn after it leaves the edge 5a of the blade 5.

[0020] A horizontal booster blade 8 is also charged to a potential of 90 kV and is positioned at the same general horizontal level as the blade 7, but on the opposite side of the blade 5. In this example, the voltage applied to the booster blade 8 is of positive polarity but it may also be of negative polarity. After the liquid leaves the applicator blade edge 5a, the liquid is charged by an electrostatic field and is atomised into droplets. The negatively charged droplets are deflected by the booster blade 8 such that the droplets travel along a path which curves upwardly to the lower surfaces of the steel strip.

[0021] The voltages to the blades 4 to 8 are supplied from high voltage transformers 12.

[0022] Both of the applicator blades 4 and 5 are at earth potential and so is the liquid supply system upstream of the blades 4 and 5. This is possible even though the liquid is electrically conductive by applying the electrical charge to the liquid after the latter has left the applicator blades, rather than as the liquid passes over the applicator blades as in the prior art.

[0023] The described electrostatic blade coater can be operated outdoors; rain would not interfere with the operation. The described electrostatic blade coater has no moving parts downstream of the metering pumps and is both efficient and safe.

[0024] Although the foregoing description has been given in terms of spraying conductive liquids, the material sprayed may include slurries and powders.

Claims

1. Electrostatic coating apparatus comprising an applicator (4, 5) for applying a coating medium, preferably an electrically conductive coating medium, to a workpiece and electrostatic charging means (6, 7) for charging the medium, characterised in that the said charging means (6, 7) is so positioned in relation to the applicator (4, 5) as to charge the said medium after the latter has left the applicator (4, 5); and that the applicator (4, 5) is substantially at earth potential.

2. Apparatus according to claim 1, characterised in that the charging means include a charging blade (6, 7) spaced from the applicator, itself in the form of an earthed blade (4, 5).

3. Apparatus according to claim 2, characterised in that the applicator blade (4, 5) has an edge (4a, 5a) in the form of a weir.

4. Apparatus according to claim 1 or 2, characterised in that the applicator is in the form of a series of spaced nozzles similar to hypodermic needles, of interchangeable size.

5. Apparatus according to any preceding claim, characterised in there is an applicator (4, 5) and a charging member (6, 7) on either side of the path of movement (X - X) of the workpiece through the apparatus.

6. Apparatus according to claim 5, characterised in that said path (X -X) is generally horizontal; the lower applicator (5) and the lower charging member (7) are supplemented by a booster charging member (8), positioned to influence the path of movement and spread of the coating medium in its passage from the applicator (5) to the workpiece to be coated.

7. Apparatus according to claim 6, characterised in that the polarity of the voltage supplied to the booster charging member (8) is the same as the polarity of the voltage supplied to the lower charging member (7).

8. Apparatus according to claim 6 characterised in that the polarity of the voltage supplied to the booster charging member (8) is opposite to the polarity of the voltage supplied to the lower charging member.

9. Apparatus according to any preceding claim, characterised in that the or each applicator (4, 5) is longer than the width of the workpiece to be coated.

10. A method of electrostatically coating a workpiece with a coating medium, preferably an electrically conductive coating medium, comprising applying an electrostatic charge to the medium, characterised by applying an electrostatic charge to the medium after the latter has been drawn off or discharged from an applicator (4, 5) and before it has reached the workpiece and maintaining the said applicator (4,5) at substantially earth potential.

Drawing