TECHNICAL TOOLBOX ISSUE 9 Continued on next page MOLD COATINGS FOR RELEASE & PROTECTION Permanent mold coating is one of the operational parameters of the casting process that is often overlooked or misunderstood. Permanent mold coatings are necessary for three basic reasons: 1. Coatings provide a protective barrier between the mold and the casting to prevent mold erosion and wear. 2. Coatings provide some degree of control over the solidification rate and direction. 3. Coatings provide a barrier between the mold and the casting so that the casting will release from the mold. JOHN HALL President CMH Manufacturing Company With proper use, a permanent mold coating can be used to control the thermal gradients such that directional solidification can be achieved. This allows a pathway for feed metal to flow into the solidifying structure and compensate for normal metal shrinkage during solidification. This is particularly important in castings with thin sections changing to think sections. The thin areas must remain open to ensure that shrinkage will not occur in the adjacent thick section. In some casting designs there might be two or more characteristics working against one another. For example, a design might have a thinwalled section in need of additional insulation to prolong solidification yet is also in an area that is difficult to release from the mold. In this case a compromise must be reached. By their very nature release coatings are not insulative and insulative coatings will not aid in the release of tight or difficult geometries. In such cases a choice must be made as to which of the two operational characteristics is most important. One choice may be to use a combination coating that will allow for some insulation and some release. Another option is to use an insulating coating as a base coat and a release top coat. Insulation coatings can vary greatly in insulating qualities as well as the surface finish the coating will impart to the casing. The insulating qualities of a coating are a function of the type of refractory filler that is used and their thermal conductivity and heat capacity. Also contributing to a coating’s insulative capabilities, as well as surface finish, is the amount of binder and the dilution rate. Typically, binders are a sodium silicate. Typical refractory materials found in mold coatings include: vermiculite, bentonite, talc, titanium dioxide, alumina, olivine, and graphite. Release and chill coatings both contain materials that act as heat conductors to allow for more rapid solidification while protecting the mold against wear. Release coatings typically contain graphite as the lubricant, which is non-wetting by aluminum. MOLD COATING APPLICATION As in any coating application, surface preparation is critical. New molds should be thoroughly cleaned. Molds that have been in service must have all the old coatings completely removed. The type of cleaning media used varies and includes sand, metal shot, grit, glass beads and dry ice (CO2). The choice depends on availability as well as how difficult the coating is to remove. In most cases it is recommended the dry ice blasting be used for routine cleaning with periodic sand blasting to restore the mold surface finish for mold coating. Over blasting, especially with sand, shot, or grit, can erode mold detail and shorten mold life. ARTICLE TAKEAWAYS: • Increase permanent mold life with coatings • Coating preparation, application and storage
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