Electrostatic Dry Powder Spray Process for Steel

The electrostatic dry powder spray process is an excellent method to use for high-volume production of parts requiring the same porcelain enamel coating. Although parts need not be identical in shape or size, it is necessary that they all be of a configuration that can be evenly coated by this process. It is a highly efficient system, with up to 99% of the porcelain enamel powder material applied to the parts; at the same time, little or no direct labor is required for application of the coating. Further savings result from the elimination of mill room and related enamel preparation operations. Also, solid waste disposal problems and a host of environmental considerations are eliminated.

In this process, powder is delivered to the spray guns from a fluidized bed feed hopper, where it is diffused by clean compressed air into a fluidlike state. The fluidized powder is then siphoned from the feed hopper by the movement of high-velocity air flowing through a venturi-type pump. Here, it is mixed with air and propelled through flexible urethane feed tubes to the spray gun. The powder feed hopper, when equipped with an automatic level control, delivers a steady, controlled flow of powder to the guns. Independent control of powder and air volume ensures proper ratios to coat the part to the desired thickness.

The powder leaves the spray gun in the form of a diffused cloud that is propelled toward the workpiece. A high-voltage, low-amperage power unit supplies the voltage to the electrode in the spray gun. When the voltage is applied, charge builds up on the electrode. The concentration of a charge creates a very strong electric field that causes the air to ionize and a corona to form. The powder is transported through this field to the grounded part, picking up charge in the area where the field is strongest. Variable voltage allows the operator to make application adjustments and to change the various application parameters.

The powder recovery booth collects the unused powder material that did not remain on the parts being coated. The collected material moves through a closed-loop system with the use of primary and secondary/final filters, ensuring that none of the powder escapes into the environment.

The development of "clean only" metal preparation systems, along with the accompanying "for clean only" ground coats, has broadened the utility and increased the use of the porcelain enamel powder process. Using this process, coating materials are available for a variety of base-coat or full-ground-coat applications such as pyrolitic surfaces, reflectors, and oven liners, as well as a full range of cover coats.

The powder coating process is used for:

• Direct-on ground-coat application over parts prepared by the "clean only" method: Here, parts requiring only a ground coat may be enameled. Also, the ground coat may be applied and fired, providing the initial part of a two-coat/two-fire process when cover coats are later applied using the wet-process method.

• Cover-coat application only: Here, powder cover coats are applied over wet-system ground coats that have been fired earlier.

• Two-coat/one-fire application: Here, a thin base coat (1 mil) is applied over a "clean only" prepared metal surface. The cover coat powder is then applied directly to the base coat powder layer at a thickness of 5 mils, making a total thickness of 6 mils. Then both the base and cover coats are fired together.

For an electrostatic powder system of maximum efficiency, a well-designed and smooth-running conveyor system is essential to avoid loss of powder from the parts prior to firing. Also, the application of the powder to parts in process should be done in enclosures where temperature and humidity are fully controlled. Very low humidity limits the buildup of the powder layer, and very high humidity causes electrical charge loss that may result in falloff. Additional details on the electrostatic porcelain enamel dry powder process are available in Porcelain Enamel Institute Bulletin P-300, "Electrostatic Porcelain Enamel Powder Application."

Porcelain enameling of cast iron is done by either the dry process or by the same wet process used for enameling sheet steel.

In the dry process, a thin coat of ground-coat enamel slip is applied to the cold casting, generally by spraying, but sometimes by dipping or other methods. After the ground coat is dry, the casting is put in a furnace and heated to a bright red heat. It is then withdrawn from the furnace. While the casting is still hot the cover coat (in the form of dry powder) is sprinkled by means of a vibrating sieve over the surfaces to be coated. The enamel melts as it falls on the hot surface. The application of powdered enamel continues until the temperature of the casting drops to the point at which the enamel will not melt. Then the piece is returned to the furnace and heated until the enamel is properly fused. For some types of products, such as lavatories, one application of powdered enamel is sufficient; other products, such as bathtubs and combination sinks, require two or more applications.

In the wet process, the enamel is applied to the part when it is cold. Handling of the part is easier, and more uniform coats of enamel are applied than in the dry process. However, because of the composition of cast iron and because of casting irregularities, it is very difficult to satisfactorily enamel large articles (such as sanitary ware) by the wet process.

During heating, gas is evolved from surface reactions of carbon in the cast iron after the temperature reaches approximately 675 °C (1250 °F). Gases produce bubbles or blisters in enamel systems that mature at temperatures higher than the temperature at which the gas evolution begins. This varies with the composition and processing history of the cast iron. Flaws in the casting act as focal points for the evolution of the gas and produce large blisters at these areas than at areas where no flaws are present.

The dry process can be used for castings of almost any size or shape. The thin ground coat allows the gas to escape from the iron during the initial firing without forming large blisters. Many castings that can be enameled by the dry process without difficulty would encounter blistering in the wet process.

Thicker coats of enamel are applied by the dry process so that the normal roughness of iron castings can be hidden with one or two applications of cover-coat enamel. These thicker coats can be fired out to a finish that has more gloss and less waviness than is common when a wet-process slip is applied by spraying to a cast iron part.

Although cast iron enameled by the wet process represents only a small portion of the total tonnage of enameled cast iron products, the wet process is used in a great variety of products and parts. These include cookware, range grates, pump impellers, and piping. Ordinarily, a dark-colored enamel whose maturing temperature is below the temperature of gas evolution is used.

Porcelain Enameling of Aluminum. Porcelain enamel slips for aluminum usually are applied by spraying, using either manual or automatic equipment with agitated pressure tanks. Slips for aluminum are not self-leveling, and therefore they must be applied smoothly in an even thickness and without runs or ripples.

Many aluminum parts are coated satisfactorily by the one-coat/one-fire method. Although the heat-treatable alloys can be recoated one or more times, the opacity and color of the coating will change with the thickness of the porcelain coating and with repeated firing. The desirable minimum fired enameled thickness is 65 pm (2.5 mils) and the desirable maximum fired thickness is 90 pm (3.5 mils).

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