3.3 Investment Casting
The investment-casting process, also called the lost-wax process, was first used during the period 4000–3000 B.C. The pattern is made of wax or plastic (such as polystyrene) by molding or rapid prototyping techniques.
Fig.3.2 Schematic illustration of investment casting
The sequences involved in investment casting are shown in Fig.3.2 The pattern is made by injecting molten wax or plastic into a metal die in the shape of the pattern. The pattern is then dipped into a slurry of refractory material such as very fine silica and binders, including water, ethyl silicate, and acids. After this initial coating has dried, the pattern is coated repeatedly to increase its thickness.[2] The term investment derives from the fact that the pattern is invested (covered) with the refractory material. Wax patterns require careful handling because they are not strong enough to withstand the forces involved during mold making. However, unlike plastic patterns (except thermoplastics), wax can be recovered and reused. When the wax pattern is melted away and the mold cavity is left, the molten metal is poured into the cavity, forming the final casting. Intricate shapes can be made with high accuracy. In addition, metals that are hard to machine or fabricate are good candidates for this process.