US3647509A - Method of producing porcelain enamel coatings - Google Patents

Abstract

A method of applying porcelain enamel to a ferrous enameling sheet stock which permits the use of an electrostatic spray in the method and which further permits eliminating the commonly required application of a ground coat of enamel and the subsequent firing. In this new method the stock is electrostatically sprayed with a flowable enamel material such as a liquid suspended frit to a thickness of a fraction only of the desired final thickness followed by drying this fractional coating slightly until it attains a self-sustaining setup condition which is nonflowing under the conditions to which it is subjected followed by hand spraying over this preliminary coating with a further coating of the enamel material to the desired final thickness and then firing the coatings to flow the enamels to a smooth glassy surface condition. This method is particularly applicable to applying two colors of porcelain enamel to ferrous stock in which one color blends smoothly into the other.

Description

United States Patent Keiser et al.

[ Mar. 7, 1972 [72] Inventors: John E. Keiser, Findlay; Earl Krieger,

Rawson, both of Ohio [73] Assignee: Whirlpool Corporation [22] Filed: May 8, 1970 [21] Appl. No.: 35,610

[56] References Cited UNITED STATES PATENTS 2,828,218 3/1958 Zimmerman ..l17/70 C 2,930,713 3/1960 Hoffman ..l17/70C Primary Examiner-Alfred L. Leavitt Assistant Examiner-Edward G. Whitby Anarney.1ames S. Nettleton, Thomas E. Turcotte, Burton H.

Baker, Gene A. Heth, Franklin C. Harter, Anthony Niewyk, Robert L. Judd and l-lofgren, Wegner, Allen, Stellman & Mc- Cord [57] ABSTRACT A method of applying porcelain enamel to a ferrous enameling sheet stock which permits the use of an electrostatic spray in the method and which further permits eliminating the commonly required application of a ground coat of enamel and the subsequent firing. In this new method the stock is electrostatically sprayed with a fiowable enamel material such as a liquid suspended frit to a thickness of a fraction only of the desired final thickness followed by drying this fractional coating slightly until it attains a self-sustaining setup condition which is nonflowing under the conditions to which it is subjected followed by hand spraying over this preliminary coating with a further coating of the enamel material to the desired final thickness and then firing the coatings to flow the enamels to a smooth glassy surface condition. This method is particularly applicable to applying two colors of porcelain enamel to ferrous stock in which one color blends smoothly into the other.

9 Claims, No Drawings METHOD OF PRODUCING PORCELAIN ENAMEL COATINGS BACKGROUND OF THE INVENTION Heretofore the use of electrostatic equipment in applying flowable enamel material directly to sheet iron or similar stock has not been commercially satisfactory because it was difficult to apply the required thickness of the deposit with the result that the enamel after firing did not have the desired appearance or was subject to chipping when stressed. The reason for this was the difficulty with electrostatic spraying of keeping the total thickness of the porcelain enamel at all areas of the metal stock within the relatively narrow limits required by acceptable quality standards to maintain the desired appearance and to prevent chipping.

In addition, it has always been difficult to produce shaded porcelain enamel and the processes used were laborious and included applying a ground coat of enamel material and firing this ground coat before the colored porcelain enamel could be applied. Furthermore, this colored porcelain enamel material had to be applied by hand. The hand application was necessary in order to prevent applying too much enamel material to the article which was of course wasteful of the material and also hand application was required to prevent producing an inferior coating of inferior appearance and excessive susceptibility to damage by chipping.

In order to overcome these problems there have been attempts to apply enamel material electrostatically. These have failed primarily because the necessary successive applications of material electrostatically form runs in the material before it had set up and it was very difficult to attain the proper shadmg.

SUMMARY OF THE INVENTION In this invention electrostatic spraying is used but only to apply a fraction of the enamel coating, (which is then permitted to dry before) applying the remainder by hand followed by drying to a self-sustaining state and then firing in the customary manner.

In this invention the entire novelty is in the method followed. The enamel materials used are strictly conventional and well understood by enamelers skilled in the art, the type of stock that is used is also well understood and the firing temperatures and techniques are likewise well known.

The ferrous enameling stock which is free from gas forming substances productive of pits and spots on subsequent treatment is described in prior publications of which, US. Pat. No. 2,602,758 is a good example. This patent also has a brief description of what is required in a proper firing temperature which, as is explained there, is the temperature at which an enamel coating becomes sufficiently fluid to flow on a metal sheet which has been prepared for the enameling to form a smooth glassy surface in an economically short length of time such as 3 to 5 minutes on a -gauge sheet.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the method of this invention the above-described stock is electrostatically sprayed with a fluid enameling material to a fraction of the desired final thickness and the remainder of the enamel material is applied by hand to the desired thickness. After this has been done the resulting composite coating is dried to a self-sustaining condition after which it is fired to produce the hard glossy enamel as discussed above. As indicated earlier, this method which eliminates the need of applying a ground coat followed by the necessary firing as practiced previously is ideally suited for making shaded colored enamel such as in producing home appliances. Thus in using the method of this invention in producing such a colored enamel coating a first area of a ferrous enameling sheet stock as described is hand sprayed with a thin continuous coating of highadherence dark-colored highlighting enamel. Then a second and adjacent area of this stock is electrostatically sprayed with a thin continuous coating of high-adherence light-colored base enamel to a desired thickness which is a fraction only such as one-half of the desired final thickness. These coatings are then dried slightly to a self-sustaining setup condition. Next the first area is hand sprayed with highlight material and the second area of the stock is hand sprayed with a thin continuous coating of a light-colored base material enamel while substantially avoiding overlap onto the first area coating. Then, the first area is again hand sprayed with a thin continuous coating, of the high-adherence dark-colored highlighting material to overlap the adjacent edge of the second area and finally the sheet stock with the enamel coatings thereon is fired as described to flow the enamels to a smooth glassy surface condition.

In a specific example of practicing the method of this invention to produce colored enamel coatings on sheet stock for use in making dryer tops a copper colored highlighting enamel and a copper colored base enamel were used having formulas and physical characteristics as follows:

Copper lbs. 3 lbs., 12 oz.

Bentonit 1 lb. 14 oz.

Silica 30 lbs. Potassium carbonate 3 lbs., 2 oz. Titanium dioxide #29 3 lbs.. 12 oz. Tris nitro 50 grams. Uverite #ZOH. 30 lbs 30 lbs.

Setit HK 1 1b., 14 oz 11b., 4 oz. Chicago Vitreous Brown Oxide #20082. 20 lbs 15 lbs. Chicago Vitreous Brown Oxide #20085. 21 lbs., 4 oz. 41 lbs., 4 oz. Chicago Vitreous Black Oxide #24492. 12 oz. 6 lbs.. 4 oz. Water 57 gallon. 57 gallon. Fineness oi grind 0-1 gram on 0-1 gram on 200 mesh 200 mesh screen. screen. Specific gravity 1.78 1.78.

The ferrous enameling sheet stock which is of the customary type having edge portions as flanges and the remainder as the face of the resulting enameled sheet was treated as follows:

Air Fluid Thick- Speclfic Pick pressure, pressure, ness Step Enamel gravity up lbs. lbs. Wei 1 Highlight- 1.78 120 15 70 .005 2..- se 1.63 65 7 45 .006 3 Highlight. 1.78 15 60 .003 4.-. Base 1.78 15 70 .004 5 Highlight- 1.76 70 5 70 .002

Pick up-the amount of enamel retained on a 1 sq. ft. cylinder (in grams).

The above Base and Highlight" formulas are themselves strictly conventional and well known.

In Step 1 the edges of the sheet corresponding to the flanges in the resulting dryer panel were hand sprayed with the highlight enamel having the characteristics given. In Step 2 the face of the sheet or the area within the boundary flanges was electrostatically sprayed with the base enamel to one-half the desired final thickness. Following Step 2 the thusly applied enamel coats were dried slightly until the coatings were selfsustaining. In this embodiment, as indicated, the drying time under heat lamps was for only 30 seconds. Then, in Step 3, the flanges were again hand sprayed with highlight material to the thickness indicated. In Step 4 the face was again sprayed with base material but this time was hand sprayed rather than electrostatically sprayed and the overspray around the edges was kept to a minimum which meant that there was substantially no overlap. in Step 5 the highlight material was again hand sprayed over the flanges and to overlap the adjacent edge of the face. In each instance the above table gives gravity, the air pressure, the fluid pressure and the thickness in fractions of an inch of the wet coating.

As indicated by the line speed of 23 feet per minute, these panels were advanced continuously in the customary manner during the indicated processing. After all the enamel had been deposited on the panels the panels were dried as indicated in an oven for 2 minutes at 375 F. As shown in the above table the final firing was in a furnace having successive zones with different and generally increasing temperatures through which the panels passed.

Having described our invention as related tothe embodiments set out herein, it is our intention that the invention be not limited by any of thedetails of description, unless otherwise specified, but rather be construedbroadly within its spirit and scope as set out in the appended claims.

We claim:

1. The method of making a smooth coating of porcelain enamel without requiring application of a ground coat and the subsequent firing thereof, comprising: electrostatically spraying a ferrous enameling sheet stock free from gas forming substances productive of pits and spots with a preliminary coating of an enamel material to a thickness of a fraction only of the desired final thickness; drying said coating slightly to a self sustaining setup condition; hand spraying said preliminary coating with a further coating of an enamel material to a desired final thickness; and firing said coatings to flow the enamels to a smooth, glassy surface condition.

2. The method of claim 1 wherein said electrostatic spraying is to a thickness of 40 to 60 percent of said desired final thickness.

3. The method of claim 1 wherein said electrostatic spraying is to a wet thickness of 0.004 to 0.006 inch and said hand spraying is to a thickness of 0.004 to 0.006 inch.

4. 'The method of making shaded smooth coatings of two superimposed colors of porcelain enamel without requiring application of a ground coat and the subsequent firing thereof,

comprising; as a first step hand spraying a first area of a ferrous enameling sheet stock free from gas forming substances productive of pits and spots with a thin continuous coating of high-adherence dark-colored highlighting enamel; as a second step electrostatically spraying a second and adjacent area of said stock with a thin continuous coating of high-adherence light-colored base enamel; as a third step drying the resulting coatings slightly to a self-sustaining setup condition;' as a fourth step hand spraying said second area of said stock with a thin continuous coating of high-adherence light-colored base material enamel while substantially avoiding overlap onto said first area coating; as a fifth step again hand spraying said first the specific area with a thin continuous coating of high-adherence darkcolored highlighting-enamel and to overlap the adjacent edge of said second area; and firing said coatings to flow the enamels to a smooth, glassy surface condition.

5. The method of claim 4 wherein said first-step spraying is to a wet thickness of 0.004 to 0.006 inch, said second-step spraying is to a wet thickness of 0.004 to 0.006 inch, said fourth-step spraying is to a wet thickness of 0.004 to 0.006

inch, and said fifth-step spraying is to a wet thickness of 0.004 to 0.006 inch.

6. The method of claim 5 wherein said firing temperature is about l,420 to 1480" F.

7. The method of making shaded smooth coatings of two superimposed colors'of porcelain enamel without requiring application of a ground coat and the subsequent firing thereof, comprising: as a first step hand spraying a first area of a ferrous enameling sheet stock free from gas forming substances productive of pits and spots with a thin continuous coating of high adherence dark colored highlighting enamel; as a second step electrostatically spraying a second and adjacent area of said stock with a thin continuous coatingof high-adherence light-colored base material enamel; as a third step drying the resulting coatings slightly to a self-sustaining setup condition; as a fourth step hand spraying said first area of said stock with a thin continuous coating of high-adherence dark-colored highlighting enamel; as a fifth step-hand spraying said second area of said stock with a thin continuous coating of high-adherence light-colored base material enamel while substantially avoiding overlap onto said first area coating; as a sixth step again hand spraying said first area with a thin continuous coating of high-adherence dark-colored highlighting enamel; and firing said coatings to fiow the enamels to a smooth, glassy surface condition.

8. The method of claim 7 wherein said first-step spraying is to a wet thickness of 0.004 to 0.006 inch, said second-step spraying is to a wet thickness of 0.004 to 0.006 inch, said fourth-step spraying is to a wet thickness of 0.004 to 0.006 inch, said fifth-step spraying is to a wet thickness of 0.004 to 0.006 inch, and said sixth-step spraying is to a wet thickness of 0.001 to 0.003 inch.

9. The method of claim 8 wherein said firing temperature is about 1,420 to l,460 F.

Claims (8)

1. 2. The method of claim 1 wherein said electrostatic spraying is to a thickness of 40 to 60 percent of said desired final thickness.
2. 3. The method of claim 1 wherein said electrostatic spraying is to a wet thickness of 0.004 to 0.006 inch and said hand spraying is to a thickness of 0.004 to 0.006 inch.
3. 4. The method of making shaded smooth coatings of two superimposed colors of porcelain enamel without requiring application of a ground coat and the subsequent firing thereof, comprising: as a first step hand spraying a first area of a ferrous enameling sheet stock free from gas forming substances productive of pits and spots with a thin continuous coating of high-adherence dark-colored highlighting enamel; as a second step electrostatically spraying a second and adjacent area of said stock with a thin continuous coating of high-adherence light-colored base enamel; as a third step drying the resulting coatings slightly to a self-sustaining setup condition; as a fourth step hand spraying said second area of said stock with a thin continuous coating of high-adherence light-colored base material enamel while substantially avoiding overlap onto said first area coating; as a fifth step again hand spraying said first area with a thin continuous coating of high-adherence dark-colored highlighting enamel and to overlap the adjacent edge of said second area; and firing said coatings to flow the enamels to a smooth, glassy surface condition.
4. 5. The method of claim 4 wherein said first-step spraying is to a wet thickness of 0.004 to 0.006 inch, said second-step spraying is to a wet thickness of 0.004 to 0.006 inch, said fourth-step spraying is to a wet thickness of 0.004 to 0.006 inch, and said fifth-step spraying is to a wet thickness of 0.004 to 0.006 inch.
5. 6. The method of claim 5 wherein said firing temperature is about 1,420* to 1480* F.
6. 7. The method of making shaded smooth coatings of two superimposed colors of porcelain enamel without requiring application of a ground coat and the subsequent firing thereof, comprising: as a first step hand spraying a first area of a ferrous enameling sheet stock free from gas forming substances productive of pits and spots with a thin continuous coating of high adherence dark colored highlighting enamel; as a second step electrostatically spraying a second and adjacent area of said stock with a thin continuous coating of high-adherence light-colored base material enamel; as a third step drying the resulting coatings slightly to a self-sustaining setup condition; as a fourth step hand spraying said first area of said stock with a thin continuous coating of high-adherence dark-colored highlighting enamel; as a fifth step hand spraying said second area of said stock with a thin continuous coating of high-adherence light-colored base material enamel while substantially avoiding overlap onto said first area coating; as a sixth step again hand spraying said first area with a thin continuous coating of high-adherence dark-colored highlighting enamel; and firing said coatings to flow the enamels to a smooth, glassy surface condition.
7. 8. The method of claim 7 wherein said first-step spraying is to a wet thickness of 0.004 to 0.006 inch, said second-step spraying is to a wet thickness of 0.004 to 0.006 inch, said fourth-step spraying is to a wet thickness of 0.004 to 0.006 inch, said fifth-step spraying is to a wet thickness of 0.004 to 0.006 inch, and said sixth-step spraying is to a wet thickness of 0.001 to 0.003 inch.
8. 9. The method of claim 8 wherein said firing temperature is about 1,420* to 1,460* F.