Grey Iron Casting Process

Several molding processes are used to produce gray iron castings. Some of these have a marked influence on the structure and properties of the resulting casting. The selection of a particular process depends on a number of factors, and the design of the casting has much to do with it. The processes using sand as the mold media have a somewhat similar effect on the rate of solidification of the casting, while the permanent mold process has a very marked effect on structure and properties.

Green sand molding is frequently the most economical method of producing castings. Until the introduction of high-pressure molding and very rigid flask equipment, dimensional accuracy has not been as good as can be obtained from shell molding. If green sand molds are not sufficiently hard or strong, some mold wall movement may take place during solidification, and shrinkage defects develop. Although castings up to 1000 lb or more can be made in green sand, it generally is used for medium to small size castings. For the larger castings, the mold surfaces are sometimes sprayed with a blacking mix and skin dried to produce a cleaner surface on the casting. This procedure is often used on engine blocks.

To withstand the higher ferrostatic pressures developed in pouring larger castings; dry sand molds are often used. In some cases, the same sand as used for green sand molding is employed, although it is common practice to add another binder to increase the dry strength.

The shell molding process is also used for making cores which are used in other types of molds besides shell molds. Its principal advantage is derived from the ability to harden the mold or core in contact with a heated metal pattern, thus improving the accuracy with which a core or mold can be made. In addition to the improved accuracy, a much cleaner casting is produced than by any other high-production process. Although the techniques and binders for hot box and the newest cold box processes differ from those used for the shell molding process, the principle is similar in that the core is hardened while in contact with the pattern.

Centrifugal casting of iron in water-cooled metal molds is widely used by the cast iron pipe industry as well as for some other applications. With sand or other refractory lining of the metal molds, the process is used for making large cylinder liners.

For some types of castings, the permanent mold process is a very satisfactory one, and its capabilities have been described by Frye. Since the cooling or freezing rate of iron cast into permanent molds is quite high, the thinner sections of the casting will have cementite. To remove the cementite the castings must be annealed, and it is universal practice to anneal all castings. The most economical composition of the iron for permanent mold castings is hypereutectic. This type of iron expands on solidification, and, because the molds are very rigid, the pressure developed by separation of the graphite during freezing of the eutectic ensures a pressure tight casting. Since the graphite occurs predominantly as Type D with very small flakes, permanent mold castings are capable of taking a very fine finish. For this reason, it finds extensive use in making valve plates for refrigeration compressors. The process is also ideally suited for such components as automotive brake cylinders and hydraulic valve bodies. Although the predominantly Type D graphite structure in permanent mold castings with a matrix of ferrite have much higher strength than sand castings of comparable graphite content, the structure is not considered ideal for applications with borderline lubrication. The castings perform very well, however, when operating in an oil bath.

Unless some special properties are desired and are obtained only with a particular casting process, the one generally selected yields castings at the lowest cost for the finished part.