US 3019126 A
Abstract available in
Claims available in
Description (OCR text may contain errors)
Jan. 30, 1962 5, A. BARTHOLOMEW 3,019,125
METHOD AND APPARATUS FOR COATING METAL STRIP AND WIRE Filed March 24, 1959 i INVENTOR & GEORGE A. BART/ 0L OMEW a Attorney United States Patent "cc 3,019,126 METHOD AND APPARATUS FOR COATING METAL STRIP AND WIRE George A. Bartholomew, Penn Township, Allegheny County, Pa., assignor to United States Steel Corporation, a corporation of New Jersey Filed Mar. 24, 1959, Ser. No. 801,626 3 Claims. (Cl. 117-17) This invention relates to the coating of metal with a protective film and, in particular, to the continuous coating of a product of indefinite length such as strip or wire.
The object of the invention is to provide a simple and effective method and apparatus for applying to a traveling product a coating of material in finely divided form.
A further object is to provide control means adapted to insure the formation of a coating of uniform thickness.
In general terms, my improved method comprises progressively heating the product to be coated and drawing it in strand form through a fluidized bed of finely divided coating material. This material may be a thermoplastic synthetic resin such as polyethylene, a refractory material such as magnesium oxide or a metal, e.g., aluminum. I employ means to heat the product to be coated immediately before it enters the fluidized bed, and after it emerges therefrom if desirable. Alternatively, I may heat the coating particles directly by the combustion of fuel in the fluidized bed. I also provide automatic means for maintaining the proper depth of the fluidized bed and the degree of fiuidization thereof. I may also utilize electrostatic force to aid the attraction of coating particles to the traveling wire or strip.
A complete understanding of the invention may be obtained from the following detailed description and explanation which refer to the accompanying drawings illustrating the present preferred embodiment. In the drawmgs:
FIGURE 1 is a flow diagram showing one form of apparatus schematically in vertical section; and
FIGURE 2 is a similar view showing a modification.
Referring now in detail to the drawings and, for the present, to FIGURE 1, I provide apparatus for coating strip or wire comprising a preheating chamber and a coating chamber 11, vertically disposed. The coating chamber includes an enlarged upper portion 12 above which are mounted radiant or induction heating units 13. Metal strip S is unwound from a coil entering chamber 10, passes between contact rolls 14 then through a fluidized bed 15 of coating-particles and, after passing between units 13, passes between heated embossing rollers 16 and is then recoiled at 17. The portion of the strip between upper and lower pairs of rolls 14 is heated by any convenient means such as radiant heaters, induction or by electric current, to a temperature of from 120 F. to 660 F. Preferable deposition temperatures are specific to individual coating compounds. Deposition temperature ranges for several classes of coating materials have been found to be:
Cellulosics About 300 F. Epoxys AboutlSO" F. Nylons About 630 F. Polyethylenes About 325 F.
3,019,125 Patented Jan. 30, 1962 ing a control valve 22. As the preheated strip ascends through the fluidized bed, the particles of coating mate rial, being heated by contact with the strip, adhere thereto and fuse together, forming a continuous coating of uniform thickness. The enlarged upper portion 12 of the coating chamber provides a settling space for any coating material which tends to follow the strip without adhering to it. Heating elements 13 effect a final curing or smoothing of the coating by maintaining it above its flow temperature. The coated strip passes between erabossing rolls 16 while the coating is still plastic and capable of being impressed with the pattern of the rolls. Thereafter the strip cools in the atmosphere to a nontacky condition before being recoiled.
I provide means for automatically controlling the flow of fluidizing air to maintain a predetermined bulk density in bed 15. For this purpose, I employ a controller 23 such as Hagan Corporation's Ring Balance Meter" responsive to the pressure difierential between points P and P in chamber 11 and to an adjustable set-point control 24. Controller 23 operates valve 22 which is a motorized valve such as the Flex Valve of Farris Flexible Valve Corporation. A similar controller 25, responsive to the pressure of the column of coating material in chamber 11, operates valve '19 of the same type as valve 22, in accordance with the adjustment of a set-point control 26.
The final coating thickness may be accurately controlled by regulation of five controllable factors: (a) strip-powder contact time, (b) preheat temperature, (c) bed density, (d) powder size, (e) flow characteristics of the powder. Contact time may be controlled by strip speed and fluid-bed height. Preheat temperature, regulated to correspond with the thermal requirements of the coating powder, is a function of heating current and strip speed and may be readily controlled over a wide range of operational conditions. Bed density is a measore of the frequency of particle-strip collisions. In conjunction with strip temperature which determines the adhesiveness of the coating particles and contact time, bed density is an effective control of coating thickness. Bed density, of course, is controlled by air-flow rate and bed depth, both of which are measured directly and automatically controlled as just explained.
Instead of air for fluidizing, I may introduce hot combustion gases into the bed 15 or a combustible mixture of fuel and air and effect combustion in the chamber itself.
In coating strip with certain materials such as Teflon plastic (polytetrafiuoroethylene), Bakelite or other thermosetting resins, which might be adversely aflected by the heat of the strip, I prefer to eliminate the preheating of the strip. In that case, I effect initial adherence of the particles to the strip by electrostatic attraction and then cause final adherence by subsequent treatment such as heating of the strip with the particles thereon. Apparatus for this process is shown in FIGURE 2 and is substantially the same as that of FIGURE 1 except for the omission of preheating chamber 10. In lieu thereof, I provide entry contact rollers 27. A source 28 of high voltage direct current is connected between rollers 27 and electrodes 30 mounted in chamber 11 adjacent the strip path. As the strip passes upwardly through chamber 11 at atmospheric temperature, the electrostatic force between the strip and the particles of coating material causes them to be attracted tothe strip and adhere thereto. As the strip emerges from the upper portion 12 of the chamber 11, the adhering particles are moderately heated by units 13 so as to fuse them into an adherent coating.
The control of electrostatic deposition is dependent upon essentially the same variables as those controlling aorarae the thermodeposition process. Strip speed, air flow, bed depth and density, contact time are all of major importance. Voltage, however, replaces preheating as the prime deposition-control factor. It is more important to control the physical and electrical properties of the powder here than in the thermodeposition process. However, except when plastic reflow is desired, thermal prop erties of the coating materials are less important in this process than in the thermodeposition process.
My process permits rapid, continuous application of accurately controlled coatings. By heating the moving strip in an enclosed chamber only a fraction of a second before coating, I effect a drastic reduction in thermal loss. By the continuous application of heat before, during and after coating, I exercise complete control of strip tem perature. Immersion time of the strip in the bed is precisely regulated by the bed height and strip speed, both of which are controlled in my process. I further control the density of the fluid bed and, therefore, have an additional processing variable which, with the alternate hotbed and electrostatic-deposition methods, supplement process flexibility and facilitate attaining the most advantageous deposition conditions.
Many advantageous coating materials such as magnesium oxide, diatomaceous earth and bentonite, are not plastic in the lower temperature range (under 650 F.)
, and may require application temperatures in other processes so high as to be detrimental to the properties of the steel base. In my process, however, such materials may I be applied to steel strip without undue harm to the base metal. This is accomplished not by heating the strip and requiring thermal transfer to colliding particles, but rather by the application of heat directly to the fluidized particles. By fiuidizing with a combination of a suitable fuel gas and air, actual combustion may be maintained within the coating bed. The excellent agitation and mixing characteristics of such beds provide almost instantaneous transfer of heat to the suspended particles, which may be heated to a plastic or even molten condition suitable for adherence to the base metal. Although the temperature within such a bed may well be considerably above the safe treatment temperature for low carbon steels, the rapid movement of the strip through the bed limits exposure to such a short time that the steel may be coated without serious overheating.
By this method, semirefractory and other inorganic and insulating-type materials may be satisfactorily applied to steel strip. Coatings similar to those now used as separating media or core-plate coatings on electrical steel may now be applied continuously and rapidly.
Many coating materials (of which Teflon plastic is an outstanding example) are seriously degraded by heating to temperatures necessary for satisfactory adhesion to the metal and cannot be properly applied by conventional coating methods. Other coating materials, e.g., polyurethanes, are produced by instantaneous chemical reaction, preferably in direct contact with the base metal.
By permitting room temperature deposition of the thermoplastic materials or through the employment of successive deposition of reactive components, the electrostatic induction feature of my process permits the use of new coating materials that, because of their thermal or chemical properties, have not heretofore been appli- Although I have disclosed herein the preferred embodiment of my invention, I intend to cover as well any change or modification therein which may be made without departing from the spirit and scope of the invention.
1. A method of coating elongated metal product which consists in drawing the product through a mass of particles of refractory coating material, fluidizing said mass by supplying a combustible mixture of gases thereto and igniting said mixture while passing through said bed hereby heating the particles of coating material to promote their adherence to the product.
2. A method of coating elongated metal product which consists in drawing the product through a mass of particles of coating material, fluidizing the mass by the upward flow of gas therethrough while maintaining an electrostatic field between the product and the particles of the mass adjacent thereto, and heating the product at a point in its travel to a temperature at which the particles remain adherent thereto.
3. Apparatus for coating a continuous metal product comprising a coating chamber, a gas-permeable bottom plate in said chamber, said plate having an opening to admit said product, guide rolls above and below said chamber whereby the product may be drawn upwardly through said opening and a mass of coating particles in said chamber, means above said chamber for heating the product after it has traversed said means, means for introducing a gas below said plate to fluidize said mass, an electrode in said chamber spaced from the path of the product and means establishing a voltage gradient between said electrode and said product.
References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENC QH Checkel: Modern Plastics, vol. 36,. No. 2, October 1958, pages 125, 126, 128, and 132 (pages 130 and 132 relied on).