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Metal Casting Technologies : September 2006
93 PORCELAIN ENAMELING Porcelain enamels are glass coatings applied primarily to products made of steel sheet, cast iron, or aluminum to improve appearance and protect the metal surface. Porcelain enamels are distinguished from other ceramic coatings by their predominantly vitreous nature and the types of applications for which they are used, and from paint by their inorganic composition and the fusion of the coating matrix to the substrate metal. Porcelain enamels of all compositions are matured at 425°C (800°F) or above. Types of Porcelain Enamels Porcelain enamels for steel sheet and cast iron are classified as either ground-coat or cover-coat enamels. Ground-coat enamels contain oxides that promote adherence of the enamel to the metal substrate. Cover-coat enamels are applied over ground coats to improve appearance and properties of the coating. The basic material of the porcelain enamel coating is called frit; it is a special glass of small friable particles produced by quenching a molten glassy mixture. Because porcelain enamels are usually designed for specific applications, the compositions of the frits from which they are made vary widely. Enamel Frits for Cast Iron. Composition of frits for enamels for cast iron vary depending on whether the frit is applied by the dry process or the wet process (Table 1). Dry process enamels are commonly used for large cast iron fixtures because of their brilliance and ability to cover small surface irregularities. Preparation of Frits. Porcelain enamel is usually applied as a suspension of finely milled frit in water; however, it may also be applied as a dry powder by electrostatically spraying on steel sheet or by dredging on cast iron. The wet-process frit is reduced to a fine powder in a ball mill. For milling, the ball charge should occupy 50-55% of the mill volume. After loading the frit charge and mill additions such as clay, bentonite, electrolytes, and coloring oxides, the water is added. Frits for dry electrostatic application are ground without water by the frit supplier and furnished to the porcelain enameler in a ready to use form. The Porcelain Enameling Process Several basic methods are used to apply the porcelain enamel to the base metal. Included are dipping, low-coating, electrodeposition, manual spray, electro-static spray, and dry- powder spray. The best method of application for a particular part is determined by quantity and quality requirements, the type of material being applied, units produced per hour, capital investment, labor cost, and, ultimately, part cost. Dipping is widely used as a method for applying the porcelain enamel, particularly when both sides of the parts require coverage. Dipping can be used for both ground-coat application and cover-coat application. It is performed by immersing the part in the prepared porcelain enamel slip, then withdrawing it and allowing the excess material to drain from the part. Flow Coating. In flow coating, the porcelain enamel slip is flowed onto the surface of the part. The process is applicable to high-volume continuous operations for parts requiring the same porcelain enamel. In automatic flow coating, the parts are placed on hangers at the correct angle for draining and are carried by conveyor through the flow coating chamber. The porcelain enamel slip is pumped at a high volume, 50 liters/ min (150 gal/min), and low pressure, 70-705 kPa (10-15 psi), through a series of nozzles that are directed at various areas of the part to ensure complete coverage. Spraying of the porcelain enamel slip is done primarily for one- side coverage. It is also used for reinforcing enamel bisque and for making repairs on enameled surfaces. Spraying is ideal for parts that are too large for hand or mechanical manipulation, particularly where service and appearance requirements permit no drain lines, beading, or buildup of the porcelain enamel. ● © Howard E. Boyer and Timothy L. Gall, eds., METALS HANDBOOK, Desk Edition, (Cleveland: American Society for Metals, 1985.) Composition, weight % Ground Cover coat coats (a) Zirconium Antimony Acid opacified opacified resistant enamel enamel enamel (a) (b) (a) and (b) SiO2 77.7 28.0 22.9 37.0 B2O3 6.8 8.8 11.2 4.9 Na2O 4.3 10.0 12.3 16.8 K2O --- 4.1 6.0 1.7 PbO 4.0 17.8 9.8 8.8 CaO --- 8.7 8.0 2.0 ZnO --- 6.1 7.5 5.9 Al2O3 7.2 4.5 6.4 1.9 Sb2O3 --- --- 13.9 13.1 ZrO2 --- 6.1 --- --- TiO2 --- --- --- 7.9 F2 --- 5.9 2.0 --- Note: (a) For dry process. (b) For wet process. Table 1. Melted-oxide compositions of frits for enamels of cast iron. Constituent WHO'S WHO OF METALS -- ANNUAL 2006/7