Grey Iron Casting Defined
Grey iron casting can be a complex process because, for iron casting manufacturers, many factors are involved in the process of dimensioning castings for iron. Particularly in regard to the casting process used and the related tooling required for grey iron casting. Design allowances must be made for casting shrinkage, draft, machining, and surface finishes. Without being familiar with metal casting processes and tolerancing practice, a design engineer could easily specify a level of quality or precision higher than the ultimate use that a cast component requires. Such over specification serves no useful purpose, but it can increase production costs substantially. Close limits on tolerances that are not critical to part function increase tooling cost, casting cost, or both without providing any benefit. Specification tolerances should be as small as necessary but as large as practical. Overly tight, unreasonable tolerances boost costs by increasing scrap and rework. In determining tolerances, consideration must be given to the limitations of a particular metal casting process. In general, green-sand molding is the least precise, dry sand molding is next, followed by shell molding, diecasting, permanent molding, and investment molding, in order of increasing accuracy. Metal tooling is more precise than wood tooling.
Although this can be complex for iron casting manufacturers, it does have its advantages. One of the chief advantages of the casting processes is that they offer the most direct ways for a grey iron casting manufacturer to manufacture finished or near-net-shape metal components. The metal casting process is usually more economical than other metal working processes because metal can be placed only where it is needed in the part, resulting in less wastage. That said, many of the economies of castings “can be lost or reduced if tolerances are not within very close, specific tolerances. So iron casting manufacturers use tabular lists of casting tolerances, to serve as general guides when manufacturing iron castings, to help limit the many variables involved in the making of a casting.
In addition to the variables inherent in molding processes and in the metal or alloy chosen are those in the coring process, individual foundry equipment and capabilities, pattern construction and condition, die dimensions at operating temperature (in the case of metal dies), heat-treating processes, metallurgical properties, and the casting design itself. If all these factors behaved exactly the same for every casting made from a given die or mold, all the castings would be exactly the same size. In the real world, however, each factor is itself a variable, and most are changing continually during a production run. The degree that these factors vary depends considerably on the sophistication of the process controls used. Nevertheless, such tolerance data are valuable for making comparisons among various processes and alloys and as starting points for discussions among casting designers, buyers, and iron casting suppliers and producers.