Precision investment casting, also known as the lost wax casting process, is an ancient metal forming technology. The technique is several thousand years old, originally producing ornaments and jewelry. Today, many high tech components are fabricated to tight tolerances using this technology.
Colloidal silica is used as an investment casting binder with refractory grains such as alumina, aluminosilicates, fused silica and zircon. These systems are used for the casting of many types of metals. NexSil 8, an eight nm alkaline sol, is a preferred binder for these systems.
In this type of casting, mould is prepared around an expandable pattern, and a single gating system is used to make a large number of casting. The pattern used to make mold is get destroyed or it completely lost its shape during investment casting. The common pattern material for precision investment casting wax, so this process is also known as lost-wax casting. The other important pattern materials used in investment casting are plastic, wood, and mercury. When mercury is used to make a pattern then the process is known as mercast.
The various steps included in investment casting
Making of pattern
The first phase of investment casting is the preparation of pattern. The individual pattern is made for each and every casting is to be made. The pattern material is wax; molten wax is injected under high pressure into the metallic die. The wax is then allowed to solidify to produce the pattern. This pattern is then ejected from die, and attached to the gating system.
Making of mould
The cluster of patterns attached to the gating system is then dipped into a slurry, made of refractory /ceramic coating material. The excess slurry is allowed to drain off from the pattern. A coating of material will formed as slurry drying. A thin shell is formed around the pattern. After curing of shell, the pattern is again dipped into slurry. The process repeats to increase the thickness of shell gradually. The required shell thickness depends on the size and shape of casting and properties of ceramic material used.
Removal of pattern from mould (Burnout)
The next step is the removal of pattern from the casting. It can be achieved by heating of mould. The wax pattern is meltdown, and it is completely drained through sprue by inverting the mould. The hot vapor of solvent such as trichloroethylene is used to remove remaining waxy residue in the mould cavity. The wax that flows down can be reused to make another pattern.
Preparing of mould and casting (Preheating, Pouring)
The mould is then preheated certain temperature (depends on the complexity of casting and material of casting and moulding). This reduces the last trace of wax and ensures proper filling of thin sections of mold, which is very difficult to achieve in cold mould. The molten metal is then poured into the mould cavity, sometimes slight pressure is preferable. After solidification, the casting takes out of the ceramic shell by breaking it or chemically dissolving it.
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