Precision Investment Casting Process - Applications, Advantages and Limitations

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.

Read: Types of Patterns Used For Casting
Types of Pattern Allowance and Its Reason

The various steps included in investment casting describe below.

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.

Applications of investment casting

This method of casting is used where high precision is required. It cannot use to make heavy casting. Investment casting largely relied on making of very intrinsic shape that cannot cast by another method. The common application includes jewelry, surgical equipment, vanes and blades for gas turbine, bolts and triggers for firearms, impeller, and stainless steel valve bodies

Advantages and Limitations of investment casting

Advantages

• It is used for casting of complex shape that cannot be produced by any other method. No limit of geometry.
• Very close tolerance and better surface finish can be achieved.
• Very fine details can be added to the pattern because pattern will remove by melting.
• The casting produced is ready to use or very little machining is required. This is useful in casting of hard material.
• In this process, controlled mechanical properties can be obtained by controlling grain size, orientation, and directional solidification. 
• Smooth surface with no parting line in casting, so there is no dimensional inaccuracy across it.
• The pattern material (wax, mercury) and sprue can be reused

Limitations

• The process is limited by mass and size. Can’t produce heavy casting.
• Expensive. Large manual operation is involved such as making of pattern and mould for each casting. The cost can be justified through less secondary finishing process.
• Difficult to cast that requires cores.
• Time-consuming process. Very long production cycle.