Machinery's Handbook, 31st Edition
SAND CASTING
1497
Sand Casting Sand casting uses natural or synthetic sand in forming molds. Larger-sized molds use green sand (a mixture of sand, clay, and some water). Advantages of Sand Casting: Ferrous, nonferrous, and even non-metal materials can be cast in this process. Sand can be reused; excess metal poured is cut off and reused as well. Simple, inexpensive tools are required. Intricate shapes can be made by this process, as molten metal flows into small sections. Finally, this process can be used to produce many small components, with no limit on the size or weight of castings. Disadvantages of Sand Casting: Accuracy and surface finish are lacking, requiring addi - tional processing and finishing of parts. Overall, sand casting is a labor-intensive process. However, since sand casting usually is the least expensive way of making a component, its cost advantage over other methods makes it an attractive molding method. Sands.—Most sand casting operations use a refractory material called silica (SiO 2 ). Sand is inexpensive and has high melting point of 3110°F (1710°C). Sand can be naturally bonded (bank sand) or synthetic (lake sand). Because its composition can be controlled more accurately, synthetic sand is preferred by most foundries. Several factors affect the selection of sand for molds. The grains of the sand must be small enough so that it can be packed densely; sand having fine, round grains can be closely packed and form a smooth mold surface. Fine-grained sand enhances mold strength, but fine grains also lower mold permeability. The sand grains must be large enough for good permeability of molds (and cores), which allows the gases and steam that evolve during casting to escape through the pores of the mold. For proper functioning, mold sand must be clean and preferably new. And the sand mold should have good collapsibility to allow the casting to shrink while cooling, thus avoiding defects such as hot tearing and cracking. Types of Sand Molds.— Sand molds are characterized by the types of sand that compose them and by the methods used to make the molds. Green-Sand Molds: Clay sand is the mixture of natural silica sand, clay, additives, and water. It is the least expensive method of making sand molds, and the sand can be recycled for the next use. Bentonite clay is used to make wet clay sand. Sand in these molds is kept moist or damp while the metal is being poured. As the wet sand has a high moisture content, good air permeability, and low strength, castings can have issues with porosity, coarse, sticky sand, and sand expansion defects. Green-sand molds are commonly used in hand molding and machine molding. In hand molding, dimensional accuracy is low, so it is generally used only for production of small and medium-sized iron castings and nonferrous alloy castings. But in mechanical mold- ing, the castings have much higher dimensional accuracy, so it is widely used for high- volume production of castings. Skin-Dried Method: In this case, the mold surfaces need to be sprayed with a mixture of 10 percent water to one part molasses or lignin sulfonate after the surfaces are dried, either by storing the mold in air or by using torches. Cold-Box Mold: Various organic and inorganic binders are blended into the sand to bond the grains chemically for greater strength. These molds are dimensionally more accurate than green-sand molds, but they are more expensive. No-Bake Mold: A synthetic liquid resin is mixed with the sand; the mixture hardens at room temperature. Because bonding of the mold in this and in the cold-box process takes place without heat, these processes are called cold-setting processes . This type of mold has good dimensional control in high-production applications. Dried Molds: Sand molds that are oven dried (baked) prior to pouring the molten metal are stronger than green-sand molds and impart better dimensional accuracy and surface finish to the casting. However, this method has drawbacks: (a) distortion of the mold is greater; (b) castings are more susceptible to hot tearing because of the lower collapsibility of the mold; and (c) production rates are slower due to the drying time required.
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