Originally published in the Jan/Feb 2019 issue of Metal Casting Design & Purchasing.
The green sand casting process is the most widely-used process for casting ferrous and nonferrous metals. The process has evolved from a manually intensive operation to a highly mechanized and fully automated process that can produce more than 300 molds per hour.
In terms of weight, complexity and alloy composition, a wide range of castings can be produced via green sand casting. In terms of shape, the process can be used to cast parts as simple as ingots and as complex as engine blocks. It is commonly used for ferrous alloys; however, it can be used for almost any alloy, including those of aluminum, copper, magnesium and nickel.
One of the major advantages of green sand casting is its low cost, especially when mass-producing castings on highly automated molding and coremaking equipment. Another major advantage of green sand is its versatility with respect to production. The process can be economically applied to small and large production runs. In addition, the production rate can be increased by making multiple mold cavities in a single mold.
When finding a casting source, there are many facts to remember about green sand.
1. The size and shape of green sand castings can have a wide range. Castings weighing from several ounces to 500 lbs. are common, while some as large as 7,000 lbs. have been made.
2. Green sand castings can be produced with walls as thin as 0.09 in. with no maximum limit. When thin walls are required, the metalcasting facility will take special precautions with respect to the shrinkage allowance in patterns, mold preparation, venting, pouring technique and other factors.
3. A tolerance of +/-0.03 in. can be cast, but such close tolerances can cause a higher rejection rate. A tolerance of +/- 0.06 in. is practical for small castings, but increased tolerances are required for larger castings.
A tolerance of +/- 0.03 in. for the first inch is possible in special cases, but at an additional cost. In general, the closest tolerances can be kept only for the dimensions that lie entirely in one part of the mold. Greater tolerances are required for dimensions across the parting line, which is subject to variations in the closing of the mold. Casting dimensions also are a function of the degree of compaction of the mold, method of pattern removal from the mold, as well as the pouring temperature and pouring rate.
4. Parts may be cast with openings or holes as small as 0.19-0.25 in. However, if required, small holes may be more economical to drill than to core. Cores may introduce some problems in a casting, such as shifting or gas porosity, and the designer should evaluate coring vs. machining options. Producing castings with undercuts and special inserts also is possible, but at an additional cost.
5. Draft on sand casting is on average 0.0625 in./ft., or 1-3 degrees. For manual molding and deeper sections, this allowance may need to be increased. In general, for the convenience of molding, draft allowances on the internal surfaces are larger than the draft allowances on the outer surfaces.
6. Typical surface finish of green sand castings is in the range of 250-1,000 micro inches, or root mean square (RMS). The surface of the castings depends on physical characteristics of the molding sand, molding method, quality of the pattern surfaces and the type of metal and pouring temperature. Since the surface finish directly influences the casting cost, designers must be careful not to overspecify an excessively smooth surface.
7. The amount of machining allowance depends on the type of metal used, casting configuration, casting size, mold characteristics, tendency of the casting to distortion and the required machining method. Small castings and castings made in large production quantities commonly will have a smaller machining allowance than large castings and castings made in small production runs. For example, typical machine finish allowance for iron castings varies from 0.1-0.4 in. for bores ranging from below 12 in. to more than 80 in. and from 0.09-0.4 in. for outside surfaces in the same dimension range. With more experience, these tolerances can be reduced.
8. Green sand castings can be heat-treated to reduce residual stresses and improve mechanical properties. Also, by placing chills in the mold, hardness and wear resistance of the casting can be increased at desired locations.
9. Like other casting processes, green sand casting has its limits. The surface finish of green sand castings is not as smooth as the finish produced from other processes. This results from the fairly rough interface between the sand mold and molten metal. In addition, tolerances in green sand castings are poorer than in other processes due to mold deformation under the metallostatic pressure and influence of heat, mold shift and unpredictability of casting shrinkage. Thus, some machining is required on castings that are used in assemblies.
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