Green Sand Casting
The typical slurry, which is what is used in green sand casting, contains 90% silica or olivine sand, 7% bentonite clay and pulverized coal and 3% water. Alternatives to this model composition include skin-dried sand which has a higher percentage of the binding agent or clay and dry sand which has no water content. Both offer improved accuracy, but only dry sand is oven baked for strengthening making it particularly useful in the production of larger parts. Green sand casting can be used to produce iron castings weighing as little as one ounce or as much as 450 tons. Typical applications include the manufacturing of machine and instrument parts such as gears, pulleys, engine blocks, cylinder heads, axles, hitches and more. This fairly simplistic method of iron casting may be used for the production of both white and grey iron parts as well as ductile iron castings and spheroidal graphite cast iron pieces, though high accuracy is difficult to achieve as is the manufacturing of identical parts as molds are used only once.
The sand molding process can be broken down into six simple steps, not including the preparation or liquification of the cast iron prior to casting. First, the sand must be shaped. This is accomplished by placing a pattern of the shape to be cast into the sand which is then placed into a bottomless and lidless box known as a flask. Cut in two, the upper flask is known as a cope while the bottom half of the mold is referred to as the drag. A gating system is needed to allow the metal to be poured in, but seal the flask during cooling. The gating system is part of step two which also entails clamping the mold to ensure it will not leak. A pre-determined molten iron alloy is then poured in to fill the cavity. The metal is then allowed to cool, an integral part of the process as this will determine the specific properties of the final part. After the desired cooling has taken place, the sand mold is broken away to remove the cast iron component. Because ejection or removal of the part from the mold involves breaking the sand away, sand casting boasts a relatively low production rate as compared to other iron castings. Nevertheless, the economic use of sand is still preferred in many foundry settings as the remnants of used molds may often be recycled for further cost savings. As the use of large grain sands can produce a rough texture on parts, iron foundries may offer secondary operations such as polishing to produce a more finished piece.
