By far sand casting is the most versatile of the various methods and technique of forming metals which include forging, punching, rolling, stamping, extrusion and many others. Sand casting affords the designer the greatest freedom and latitude of any forming methods with an unlimited choice of metals and alloys that can be readily sand cast singly or by millions. Sand casting are produced in a wide range of sizes form a fraction of an ounce of over 100 tons
The first stage in the production of sand castings must be the design and manufacture of a suitable pattern. Patterns for sand casting have traditionally been made of wood or metal. And the pattern has to be made larger than the finished casting size to allow for the shrinkage that takes place during solidification and cooling. The extent of this shrinkage varies with the type of metal or alloy to be cast.
Green sand generally consists of silica sand and additives coated by rubbing the sand grains together with clay uniformly wetted with water. More stable and refractory sands are used, such as fused silica, zircon, and mullite, which replace lower-cost silica sand and have only 2% linear expansion at ferrous metal temperatures. Also, relatively unstable water and clay bonds are being replaced with synthetic resins, which are much more stable at elevated temperatures.
Sprues, Runners, and Gates
Access to the mold cavity for entry of the molten metal is provided by sprues, runners, and gates. A pouring basin can be carved in the sand at the top of the sprue, or a pour box, which provides a large opening, may be laid over the sprue to facilitate pouring. After the metal is poured, it cools most rapidly in the sand mold. Thus the outer surface forms a shell that permits the still molten metal near the center to flow toward it. As a result, the last portion of the casting to freeze will be deficient in metal and, in the absence of a supplemental metal-feed source, will result in some form of shrinkage. This shrinkage may take the form of l shrinkage (large cavities) or the more subtle microshrinkage (finely dispersed porosity). These porous spots can be avoided by the use of risers，which Provide molten metal to make up for shrinkage losses.
Pouring the Metal
The pouring process must be carefully controlled, since the temperature of the melt greatly affects the degree of liquid contraction before solidification, the rate of solidification, which in turn affects the amount of columnar growth present at the mold wall, the extent and nature of the dendritic growth, the degree of alloy burnout, and the feeding characteristics of the gating system.
After the castings have solidified and cooled, they are placed on a shakeout table or grating on which the sand mold is broken up, leaving the casting free to be picked out. The casting is then taken to the finishing room where the gates and risers are removed. Unwanted metal protrusions such as fins, bosses, and small portions of gates and risers are smoothed off to blend with the surface. The casting after grinded is sent into the shot-blasting equipment, in where the oxide skin, oil stain, sand sticking, etc. were removed. Meanwhile, which can make the castings show even and consistent metal color.
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