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The use of shell-mold casting has grown significantly, because it can produce many types of castings with close tolerances and good surface finishes at a low cost.

Fabrication of a Shell Mold

In this process, a mounted pattern, made of a ferrous metal or aluminum, is heated to 175-370 degrees Celsius, coated with a parting agent such as silicon, and clamped to a box or chamber containing a fine sand containing a 2.5-4.0 percent thermosetting resin binder, such as phenol-formaldehyde, which coates the sand particles. The sand mixture is blown over the heated pattern, coating it evenly. The assembly is often -- but not always -- placed in an oven for a short period of time to complete the curing of the resin. The shell hardens around the pattern and is removed from the pattern using built in ejector pins. Two half-shells are made in this manner and are bounded or clamped together in preparation for pouring.

Figure 1: Heated pattern placed over a dump box containing a sand and resin mixture.

Figure 2: The box is inverted and a shell partially cures around the pattern.

Figure 3: The box is righted.

Figure 4: The top is removed and the shell is further cured and is finally stripped from the pattern

Once the shells are formed, matched shells are joined and supported in a flask ready for pouring

Properties of a Shell Mold

The shells are light and thin, usually 5-10 mm, and consequently their thermal characteristics are different from those for thicker molds. The thin shells allow gases to escape during solidification of the metal. The mold is generally used vertically and is supported by surrounding it with steel shot in a cart. The walls of the mold are relatively smooth, offering low resistance to flow of the molten metal and producing castings with sharp corners, thinner sections, and smaller projections than are possible in green-sand molds. With the use of multiple gating sytems, several castings can be made in a single mold.

Source: Kalpakjian, Manufacturing Processes for Engineering Materials.