METAL CASTING APPLICATIONS
Clinton L. Atwood
Investment casting, often called lost wax casting, is regarded as a precision casting process to fabricate near-net-shaped metal parts from almost any alloy. Although its history lies to a great extent in the production of art, the most common use of investment casting in more recent history has been the production of components requiring complex, often thin-wall castings. While a complete description of the process is beyond the scope of the discussion here, the sequential steps of the investment casting process will be briefly described, with emphasis on casting from rapid prototyping patterns.
The investment casting process begins with fabrication of a sacrificial pattern with the same basic geometrical shape as the finished cast part (Fig. 10.1a). Patterns are normally made of investment casting wax that is injected into a metal wax injection die. Fabricating the injection die often costs tens of thousands of dollars and can require several months of lead time. Once a wax pattern is produced, it is assembled with other wax components to form a metal delivery system (Fig. 10.1b), called the gate and runner system. The entire wax assembly is then dipped in a ceramic slurry, covered with a sand stucco (Fig. 10.1c), and allowed to dry. The dipping and stuccoing process is repeated until a shell of ~6-8 mm (1/4-3/8 in) is applied.
Once the ceramic has dried, the entire assembly is placed in a steam autoclave to remove most of the wax (Fig. 10.2a). After autoclaving, the remaining amount of wax that soaked into the ceramic shell is burned out in a furnace (Fig. 10.2b). At this point, all of the residual pattern and gating material is removed, and the ceramic mold remains. The mold is then preheated to a specific temperature and filled with molten metal, creating the metal casting (Fig. 10.3a). Once the casting has cooled sufficiently, the mold shell is chipped away from the casting. Next, the gates and runners are cut from the casting, and final postprocessing (sandblasting, machining) is done to finish the casting (10.3b). Fig. 10.4 shows the CAD solid model, the shell, and the pattern produced in the QuickCast process.
Fig. 10.3. Investment casting process description (cont'd.)
Fig. 10.4. CAD solid model (top), ceramic shell with QuickCast pattern (left), QuickCast pattern (right).
The major impact rapid prototyping processes have had on investment casting is their ability to make high-quality patterns (Fig. 10.5) without the cost and lead times associated with fabricating injection mold dies. In addition, a pattern can be fabricated directly from a design engineer's computer-aided design (CAD) solid model. Now it is possible to fabricate a complex pattern in a matter of hours and provide a casting in a matter of days. Investment casting is usually required for fabricating complex shapes where other manufacturing processes are too costly and time-consuming. Another advantage of rapid prototyping casting is the low cost of producing castings in small lot sizes.