Formation of coatings

Dec. 30, 1969 w. bullough 3,486,484

FORMATION OF COATINGS

Filed Sept. 19, 1966 4 Sheets-Sheet 1

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William Bullough

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ATTORNEYS

Dec. 30, 1969 w. bullough 3,486,484

FORMATION OF COATINGS

Filed Sept. 19, 1966 4 Sheets-Sheet 2

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INVENTOR

W/ujam Bullough

BY

ATTORNEYS

W. BULLOUGH

FORMATION of coatings

Dec. 30, 1969

Filed Sept. 19, 1966

3,486,484

Sheets-Sheet 3

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-'•.INVENTOR

WlLLIAM BULLOUGrH

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ATTORNEYS.

W. BULLOUGH

FOBMATION OF COATINGS

Dec. 30, 1969

Filed Sept. 19, 1966

3,486,484

Sheets-Sheet 4

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INVENTOR

ATTORNEYS

3,486,484

Patented Dec. 30, 1969

United States Patent Office

3,486,484 FORMATION OF COATINGS

William Bullough, Mayals, Swansea, Glamorgan, Wales, assignor to The British Iron ‘ and Steel Research

Association ⁵

Filed Sept. 19, 1966, Ser. No. 580,527

Claims priority, application Great Britain, Sept. 23, 1965, 40,684/65

Int. Cl. B05b 5/02

U.S. Cl. 118—636 4 Claims io

ABSTRACT OF THE DISCLOSURE

An apparatus for depositing a uniform layer of pow- _ dered material upon a moving substrate as a preliminary ¹⁰step in a coating process. The powder is released from the bottom of a container and pressed by a pressure plate into circumferential grooves in a horizontal rotating cylinder. Means are provided for scraping the powder from the grooves as the cylinder rotates and into a vibrating screen ²⁰system beneath the cylinder. The vibrating screens break up any agglomerates of powder and deposit the powder in a uniform layer on the substrate moving beneath said screens.

This invention is concerned with the formation of coatings by powder deposition on a substrate.

Metal sheet materials having a coating on one or both surfaces thereof of another metal or alloy, a metal oxide ₃₀or a plastics material are required for many purposes. The present invention is concerned with a process for the production of such coated metal sheet materials in which a layer of powdered material, which is to form the eventual coating, is deposited on a metal strip and is subsequent- _3gly rendered coherent in itself and adherent to the metal strip, for example by compaction followed by sintering. Tn order to obtain satisfactory coatings by such a process it is essential that the powder coating should be substantially completely uniform, i.e. in terms by weight of pow- 40 der per unit area of the surface of the substrate and the devices at present available for applying powder coatings to strip materials do not enable powder coatings of a satisfactory degree of uniformity to be obtained.

We have now devised an improved apparatus for de- ^_gpositing a coating of a powder on a moving strip. According to the invention, there is provided a rotatable metering roll having a length at least equal to the width of the strip to be coated and provided, on its curved surface, with a plurality of circumferential grooves, a container _ggfor powder having a powder outlet arranged to supply powder to said grooves along the length of the roll, means for pressing powder into said grooves, scraper means for removing excess powder from the curved surface of the roll adjacent the said grooves, means for removing the ₅₅powder from said grooves, and means for breaking up the powder agglomerates removed from said grooves and depositing the powder substantially uniformly on the strip.

Further features and advantages of the invention will become apparent from the following description of em- θθ bodiments of coating apparatus, given by way of example only, with reference to the accompanying drawings in which:

FIGURE 1 is a diagrammatic representation of apparatus according to the invention; 35

FIGURE 2 shows diagrammatically a modified form of screen;

FIGURE 3 is a perspective diagrammatic view of an apparatus as shown in FIGURE 1; and

FIGURE 4 is a vertical view in partial section of γθ another screen arrangement of the present invention.

Referring to FIGURE 1, the apparatus comprises a rotatable metering roll 10 provided with a plurality of circumferential grooves 11, a hopper 12 for the powder to be deposited situated above the roll 10 and a bank of wire screens 13 which can be vibrated, situated below the roll 10 and above the strip 14 to be coated. The hopper 12 extends across the length of the roll 10 and is provided with a pressure plate 15, also extending across the length of the roll, which serves to press the powder into the grooves 11. The hopper is also provided with a plate 16 which, in use, extends into the powder and is vibrated; this vibrating plate serves to ensure a continuous supply of powder to the roll and to prevent bridging of the powder in the hopper. The lower end of the plate 16 is preferably perforated.

Tn use the roll 10 is rotated in an anti-clockwise direction (referring to the drawing) and associated with the roll are two scrapers 17 and 18 which extend across the length of the roll and serve to remove powder from the surface of the roll between the grooves 11 and level off the powder in the grooves, after each portion of the roll has passed under the hopper. A number of discharge blades 19 equal to the number of grooves 11 are provided; the blades 19 are of approximately the same width as the grooves 11 and are arranged to bear against the bottom of the grooves and they serve to remove the compressed powder completely from the grooves in the form of small pellets.

In use, the roll 10 is rotated at the speed required to give the desired thickness of powder coating on the strip 14, the thickness of the coating also being determined by the roll diameter and the width, depth and spacing of the circumferential grooves. As a guide to suitable speeds of rotation, we have established the following empirical relationship for a metering roll having a diameter of 15 cm. and provided with circumferential grooves 1.65 mm. wide and 1.58 mm. deep uniformly spaced 6.35 mm. apart.

peripheral roll speed gg 5 = strip speed coating thickness in microns

As the roll rotates, the powder is pressed into the grooves by the pressure plate 15 and the excess powder is removed by the scrapers 17 and 18. The discharge blades 19 scrape the powder out of the grooves and the powder falls under gravity, in the form of small pellets, through the bank of vibrating wire screens 13 where the pellets are broken up and the resulting powder then falls onto the moving strip 14.

The pressure plate 15 and the scrapers 17 and 18 are suitably made of Phosphor bronze and the pressure plate 15 is advantageously perforated across its width to allow the excess of powder drawn in by the roll to pass back into the hopper.

The screens 13 are vibrated by any suitable means, such as a compressed air-actuated rotating ball vibrator 20. The displacement of the screens during vibration is preferably of high frequency and low amplitude. The screens 13 may take a number of forms: in one arrangement a series of 1.58 mm. steel wires were mounted in 1.58 mm. thick side plates. The wires were spaced 5 mm. apart horizontally and 2.5 mm. apart vertically. They were arranged to form two banks, the top one consisting of seven rows and the bottom eighteen rows of wires. Alternate rows were offset in respect to the row above resulting in a grid where the gap between the wires was 1.58 mm. The two banks of wires were 3.2 cm. apart. Overall dimensions of the grid were 19 cm. long, 16 cm. wide and 12 cm. deep. The grid was arranged beneath the roll so that the wires were across the strip, the top row of wires being 10 cm. from the

3,486,484 bottom of roll and the bottom row 3.18 cm. from the strip. The grid was attached to two steel rods supported near their ends by ebonite blocks and were free to move horizontally in the direction of movement of the strip by sliding in nylon bushes pressed in the ebonite; the rods were connected to the vibrator. The dimensions just quoted are those suitable for use with a metering roll of the dimensions mentioned above; in the case of metering rolls of other dimensions, the dimensions of the vibrating grid must, of course, be suitably altered.

In another arrangement of the screen 13, see FIGURE 2, a series of concentric, semicircular, expanded metal grids are carried by spaced ebonite blocks 30, the concave surfaces of the grids being directed upwardly. The diameter of the inner grid is 11.43 cm. and that of the outer grid is 21 cm. The blocks are spaced 20.3 cm. below the bottom of the metering roll and 10.5 cm. above the surface of the strip, the lowest point on the outer grid being about 2.54 cm. from the surface of the strip. The blocks in turn are carried by ebonite rods 31 supported by flexible polythene supports 32 spaced approximately 50.8 cm. apart and permitting horizontal movement of the screen in the direction of movement of the strip; the rods are connected to the vibrator. The above dimensions are those suitable for use with a 22.8 cm. diameter metering roll.

To prevent “hold up” of powder in the grids a series of .635 cm. diameter steel rods 34 are positioned within the screen. The rods 34 are suspended by light springs fixed to polythene supports and are arranged so that there is a rod resting on each grid. The polythene supports are attached to the blocks 30; thus when the screen is vibrated the rods bounce against the grids and prevent the “hold up” of powder.

The disintegration of the powder pellets falling from the roll can be assisted by applying a high tension electric potential between the screens 13 and the strip 14; suitable potentials are, for example, from 15 to 20 kv. The application of this voltage to the screens causes the powder particles to acquire charge on their surface and to be repelled with increased velocity from the charged wires towards the earthed strip. This reduces the effect of draughts on the powder and due to the fact that each particle is charged, repulsion occurs between the particles resulting in greater dispersion and uniformity than can be obtained without the applied voltage.

When the screens 13 are charged the metering roll may be either charged or earthed; in the former case the metering roll should be charged to a higher potential than the screens.

Where both the strip and the metering roll are earthed, the screen or screens should be nearer the strip than the metering roll so that the field strength between the screen(s) and the strip will be higher than that between the screen(s) and the metering roll; this has the effect of minimising the tendency for the powder particles to deposit on the metering roll once they have been charged by the screen(s).

The apparatus is particularly suitable for depositing metal powders, more particularly aluminum and nickel powders, although it may also be used for depositing other types of powder such as zinc, stainless steel and possibly titanium powder. When used for depositing metal powders, the grooves in the metering roll are suitably about 1.5 mm. wide and deep and are spaced about 5 mm. apart. For materials such as powder plastics materials, the grooves should normally be made larger in view of the larger particle sizes of such powders.