Role of Coating Thickness Firing Time and Temperature Metal Substrate and Color

The properties and performance of the final porcelain enamel surface are markedly affected by the thickness of the applied coating, the firing time and temperature, and the selection and fabrication of the metal substrate. Color matching and control are often vital to the quality of the product or part.

Coating Thickness. The optimum thickness of porcelain enamel depends on the substrate metal and the service requirements of the part. In any application, however, the enamel coating should be applied at the minimum thickness to achieve the desired appearance and functionality.

On sheet steel, a wet ground coat 50 to 100 pm (2 to 4 mils) thick is applied to promote adhesion. To cover the ground coat, a very opaque white or pastel cover coat 100 to 150 pm (4 to 6 mils) thick is required. Thus, a two-coat wet system has a thickness ranging from 150 to 255 pm (6 to 10 mils). Brightly colored porcelain enamels are produced by applying less-opaque coats with more saturated colors over a white intermediate coating. For these, the total thickness of the coating system should range from 200 to 350 pm (8 to 10 mils). Some decorative finishes are textured; the thickest parts of these coatings may be up to 635 pm (25 mils). Coating thickness of 125 to 150 pm (5 to 6 mils) is required for covercoat porcelain enamels applied directly to decarburized or to specially stabilized steels.

The thickness of wet porcelain enamel on a large steel part of simple configuration can be closely controlled when application is by a mechanical spraying system that is adapted to the part. For example, mechanically applied porcelain enamel on curved silo panels 2 by 3 m (5 by 9 ft) can be maintained within ±13 pm (±0.5 mil); however, when application is by hand spraying, the variation in enamel thickness may be as much as ±50 pm (±2 mils).

On aluminum, porcelain enamel is applied to produce a fired enamel thickness ranging from about 65 to 100 /'m (2.5 to 4 mils). A tolerance of ±13 pm (± 0.5 mil) is required for a white enamel coating 65 pm (2.5 mils) thick, to achieve acceptable opacity.

Coatings for cast iron products are much thicker than those for sheet steel or aluminum. Dry process coatings on cast iron products, such as sanitary ware, range from 1020 to 1780 pm (40 to 70 mils) in thickness. Coatings applied to cast iron by the wet process range from 255 to 635 pm (10 to 25 mils) in thickness.

Firing Time and Temperature. Firing of porcelain enamel involves the flow and consolidation of a viscous liquid and the escape of gases through the coating during its formation. Within limits, time and temperature are varied in a compensating manner. For example, similar properties and appearance may result when a coated steel part is fired at either 805 °C (1480 °F) for 3 1 min or at 790 °C (1450 °F) for 4 min. Of course, there is a minimum practical temperature for the attainment of complete fusion, acceptable adherence, and desired appearance. Most wet ground-coat enamels for high-production steel parts exhibit acceptable properties over a firing range of 55 °C (100 °F) at an optimum firing time. However, control within 11 °C (20 °F) is ordinarily maintained to produce uniform appearance and to allow interchangeability of parts. The combined effects of increased firing time and temperature result in more thorough firing, and, up to a maximum, the following conditions occur:

• Colors shift dramatically, particularly reds and yellows. In general, white and colors shift toward yellow.

• Gloss of the enamel coating changes.

• Chemical resistance of the enamel coating increases.

• Gas bubbles are eliminated.

• Enamel coating becomes more dense and brittle and less resistant to chipping.

• Maximum adherence is attained in the optimum portion of the firing range.

Avoiding Metal Distortion. Sag and distortion of sheet metal parts result from low metal strength at the firing temperature, thermal stresses due to nonuniform heating and cooling, and transformation to austenite. Changes in design of the parts and firing practice alleviate the first two causes, and the use of extra-low-carbon content or of special stabilized steels minimizes transformation from ferrite to austenite.

Ground-coat enamels do not contribute significantly to any distortion of parts because their coefficients of thermal expansion approach that of steel. When ground coat is applied to both sides of the metal, there is a counterbalancing of expansion and contraction stresses.

The effect of cover-coat enamels on the flatness of porcelain enameled parts can be pronounced because of their lower coefficients of expansion and their one-side application. The likelihood of distortion is greatly increased when multiple or thicker coats of cover-coat enamels are necessary on one surface. Sometimes cover coats must be applied to the back side of parts to equalize the stresses.

Adjustments in the firing cycle can sometimes help minimize distortion. A cycle with relatively slow heating and cooling rates is preferable to rapid heating and cooling.

Variations in the method of supporting the work during firing can often reduce the sagging characteristics. Furnace supports and fixtures can be designed to distribute the load and equalize heating and cooling rates. Porcelain Enamel Institute Bulletin P-306, "Design and Fabrication of Sheet Steel Parts for Porcelain Enameling," has more information.

Color Matching and Control. In color matching, primary coloring oxides are normally used. In most instances, two or three oxides are sufficient to match any specific color. A minimum number of oxides should be used; for example, a stable green oxide is preferred to a blend of blue and yellow oxides. Usually, the proper color intensity is obtained first; then adjustment is made for the desired color shade.

Color stability can be adversely affected by improper mill additions. However, a color with only fair stability may be improved by the proper mill additions, and minor color adjustments.

Finer grinding reduces the intensity of the color. It is imperative that the fineness of the milled color be controlled within specified limits. The thickness of the fired enamel coating affects many colors. In general, thick coatings produce lighter colors and thin coatings result in darker colors.

The set and specific gravity of a colored enamel slip are important to the finished results. Color corrections of electrostatic dry powder cannot be made by the enameler.

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