The use of low-boiling solvents, low air pressure and heated-hose delivery, allow spray painting jobs to be carried out quickly and effectively withou t incurring the drawbacks of high-pressure hot paint spraying. Aerograph-DeVilbiss Co. Ltd . of Bournemouth have released a general description of their new heated paint system which is said to bring a fresh approach to the advantages which can be gained from paint heating, and is of individual design. Its advantages are to be felt in production, maintenance, or decorative painting in applying a wide range of materials to most surfaces, including steel and timber. The idea of heating paint is not new. For generations painters have known that if they were to stand their paint in hot water it would become thinner and easier to brush. A thorough study of what happens when paint is heated, and how these changes can be exploited in spray painting, has given rise to the new concept of low- pressure hot spray. Method of heating paint Initially water is heated in a separate electric unit. It is then piped to the heat exchanger where the paint itself is heated (see diagram). The paint passes through ribbon-like passages which are surrounded by constant-temperature water jackets. The heated paint is transferred to the gun through a heated hose, and the circulating hot water system ensures that the material is never in contact with the heating element, so that there is no the spray gun because of the lower viscosity and because a risk of the paint hardening. greater percentage of solvents is evaporated by heat. Paint is saved because it is no t carried off with the solvents. High boiling Theory solvents remain to cause flow out, resulting in a better finish. Thi s low-pressure hot spray allows th e use of materials thinned Results in exactly the same way as for cold spraying. In any paint application thinners only help in getting the coating on the Greater paint mileage and film build are achieved, possibly u p surface. The final worth of a coat of paint lies in the amount to 50% and rarely below 20%. The improved flow out of and character of solid deposit remaining after complete curing heated materials produces smoother, more glossy finishes or drying has taken place. It is essential, then , to consider what because there is far less tendency towards such common happens to paint solids and solvents both in cold spraying and problems as orange peel, fatty edges, sags, and runs. Production hot spraying. speed is increased in jobs needing several coats because they are generally completed in fewer applications. With cold spraying, if a material of 20-30 seconds viscosity (No . 4 Ford Cup) is sprayed, it is found that a high air pressure Another very important feature is that seasonal temperature is needed to atomise the paint into a sufficiently fine spray for changes have no effect on the viscosity of a material being applied successful application. This high pressure, however, rapidly by hot spray, and, particularly relevant, neither have exterior evaporates solvents which simultaneously carry off paint solids and room temperature changes throughout the working day. with them. Therefore, less of the solids and correspondingly These can lead to uneven film build and persistent runs and sags less film build are deposited on the surface in one coat. in spite of frequent gun and pressure adjustments. With a paint heater, if wide temperature changes do occur, th e atomisa- I n low-pressure hot spray low-boiling solvents are used for tion and spray pattern are unaffected because the paint heater thinning the material. A 20-30 second viscosity material (No. 4 Ford Cup) when applied by this method will give a lower delivers the fluid at the same controlled temperature, and, viscosity material reduced by heat. Low air pressure is used at therefore, the same viscosity. September 1965 CORROSION TECHNOLOGY 29 D2
Anti-Corrosion Methods and Materials – Emerald Publishing
Published: Sep 1, 1965
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