Mold coatings should be sprayed onto the mold surface with an airless spray gun or an aspiration type spray gun. Spraying equipment may be one of many different styles and types of spray guns. Use of a paint gun is not recommended as the heavy materials in the mold coating easily clog the small ports. A siphon type gun that has one straight fluid tube with replaceable fluid tips works well. Some siphon guns are available with interchangeable pots. With extra pots, two or three different types of coating may be kept on hand, mixed and ready. As the need arises for a particular coating, it may be snapped on the spray head and used immediately. In areas where a high degree of insulation is required, such as gates, runners, risers and pouring cups, brushing the coating on will provide more insulating capability. In addition to the insulating properties of the coating itself, brushing will trap air bubbles, which enhance insulation. Additionally, the rough surface caused by brushing can aid in molten metal flow through the gating system by continuously disrupting the oxide skin as the metal flows. Thoroughly mix the coating in its original container before diluting or using only a portion of the can weight. This will alleviate any settling problems that can occur during transit and storage. When diluting, soft warm water works best, but cold water is acceptable. In either case, adequate mixing with any equipment such as a Lighting mixer or a bent rod in a hand drill is required. Excessive shear should be avoided. The mold should be heated to 600°F (315°C). Care must be taken to heat the mold uniformly. Optical pyrometers should be used to determine if the mold is heating evenly. While the mold is hot it should be sprayed lightly with water. This will increase a porous oxide film on the mold, which will provide a good surface for the mold coating to bond to. The water spray also cools the mold to the desired coating application temperature, 350° - 400°F (75° - 200°C). If the molds are too hot, the rapid expanding water vapor front moving away from the mold will cause a phenomenon known as “kick back,” and very little coating will adhere to the mold. Even the coating that does adhere will not be properly bonded. If the mold is too cold the coating might run resulting in an uneven surface. Depending on the brand of coating, a primer coat may be required. This could be a specifically designed primer or a diluted version of the main coating. The purpose of the primer coat is to create the best possible adherence of the coating to the mold. This occurs because very diluted sodium silicate solutions allow for bonds that are more parallel to the mold face. This structure forms a stronger bond, which is more resistant to wear. In contrast, high sodium silicate solutions create bonds that are perpendicular to the mold face and can be sheared off. Care must be taken not to over dilute the primer coating, as sufficient sodium silicate must be present to generate the bond. Once the primer is applied, the main coating can be applied at higher concentrations. Do not try to cover the mold face with one heavy coating. A gradual buildup of the coating is preferred over one heavy coat. The number of coats and the exact coating thickness will vary with the casting design and may vary within the mold itself. A working profile should be developed for where a heavier or thinner application of coating should be applied to aid solidification. After the coating has cured, excess coating should be removed from the parting line and core prints with a wire brush or soft brass scraper. As noted, most all commercial mold coating materials are bonded by sodium silicate with various filler materials for their insulative, lubricative or cosmetic qualities. STORAGE Mold coating is supplied in fivegallon cans or fifty-five-gallon drums. The coatings should be stored in their original covered containers with the lids firmly in place when not in use. Mold coating should be stored in a dry place away from excessive heat or cold or drastic temperature change. Ideal storage temperatures range from 50°- 75°F (10°- 25°C). Under no circumstances should the coating material be allowed to freeze, as subsequent thawing may not restore the coating to its original condition. Refer to the coating manufacturer’s instructions for additional storage information. Contact: JOHN HALL [email protected] 10
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