Investment Casting

Investment casting produces very high surface quality and dimensional accuracy. Investment casting is commonly used for precision equipment such as surgical equipment, for complex geometries and for precious metals. This process is commonly used by artisans to produce highly detailed artwork. The first step is to produce a pattern or replica of the finished mould. Wax is most commonly used to form the pattern, although plastic is also used.

Patterns are typically mass-produced by injecting liquid or semi-liquid wax into a permanent die. Prototypes, small production runs and specialty projects can also be undertaken by carving wax models. Cores are typically unnecessary but can be used for complex internal structures. Rapid prototyping techniques have been developed to produce expendable patterns. Several replicas are often attached to a gating system constructed of the same material to form a tree assembly. In this way multiple castings can be produced in a single pouring (Jain, 1986). The next stage is to create a one-piece destroyable mould around the pattern. This mould is built up around the wax pattern in stages by alternately coating the assembly with a specially formulated heat-resistant refractory slurry mixture and then applying a granulated refractory 'stucco' shell.

The initial coats use a fine powder, which creates a very smooth and dimensionally accurate negative of the pattern. Subsequent coats use a coarser refractory material to build up sufficient thickness. This material hardens around the assembly at room temperature. This investment shell casting method is the more common process.

An alternative process is to use an investment flask, where sand is packed around the mould. This can be desirable where additional mould strength is required and also allows the casting size to be increased. In both shell and flask casting, the pattern is removed from the mould prior to the pouring stage. The mould is inverted and heated to melt and remove the wax.

In some operations the melted wax is recovered and reused to make new patterns. After multiple reuses the material needs to be reconditioned to maintain its purity, or disposed of.

The mould is then heated in an oven to remove any residual wax and to further cure and harden the mould. The temperature is raised to 980oC prior to pouring. This is a time- and energy-consuming process: total heating time, from wax removal to pouring, can take up to 15 hours (Jain, 1986). Molten metal is then poured into the central cavity and flows into the individual moulds. After the metal has cooled, the mould material is removed.

Because of its very high dimensional accuracy the Investment Casting process can achieve a netshape cast requiring little or no machining. Great care is taken in the patternmaking stage to remove any mould lines because it is more cost effective to remove unwanted material from the wax model than from the final cast.

Advantages of Investment Casting