Casting investment to crack

2010-06-19T22:23:00.000-07:00

There are several faults which can occur in alloy castings, most of which can be traced to incorrect selection of materials or faulty technique.

Investment mould

The mould cavity is formed by allowing the investment to set around the wax pattern and associated sprue former and spruce base (crucible). After allowing the investment material to set, the sprue base and former are removed and the wax pattern ‘burnt out’ to leave the completed mould cavity. The choice of investment material depends on the type of alloy which is to be cast. The casting ring liner serves a dual purpose. It forms a relatively pliable lining to the inner surface of the rigid metal casting ring. This allows almost unrestricted setting expansion and thermal expansion of the investment. In addition, its thermal insulating properties ensure that the investment mould does not cool rapidly and contract after removal from the ‘burn out’ oven.

Casting machines

Numerous types of casting machines are available, the aim of each being to cause molten alloy to completely fill the investment mould cavity. The three main variables which characterise the machines are as follows:

The alloy may be melted in the mould sprue base (mould crucible) or in a separate crucible located in the casting machine.
The alloy may be melted by one of several methods including gas-air torch, oxyacetylene torch, electrical induction heating or electrical resistance melting.
The molten alloy may be driven into the mould by gravity, air pressure, steam pressure or by centrifugal force. The arm of the centrifugal casting machine is rotated either by a spring or by means of an electric motor.

Faults in castings

The faults which can occur in casting may be of four types.

Finning and bubbling.
Incomplete casting.
Porosity in casting.
Oversized or undersized casting.

Finning and bubbling: Finning occurs when the investment is heated up too rapidly in the furnace. This causes the investment to crack. Bubbling effects on casting appear as spheres of excess material attached to the surface of the casting.

Incomplete castings: There are many possible causes of incomplete castings. In any casting the greater the number and thickness of the sprues, the more readily the metal will fill the mould.

Porosity: Porosity may be seen as surface pitting on the casting or may be revealed within the cast metal on finishing and polishing. Broken pieces of investment, or particles of dirt which have fallen down the sprue, may become embedded in the casting and produce pitting of the surface. For this reason all casting moulds should be handled with the sprue downwards.

Undersized or oversized castings: The final fit of a casting depends on a balancing out of expansions and contractions which occur during its construction. The major dimensional changes involved are the casting shrinkage of the alloy which should be compensated for by the setting expansion, thermal expansion and inversion of the investment. Faults in technique, for example not heating the investment mould to a high enough temperature, may produce insufficient compensation for casting shrinkage. However, that other factor such as the choice of impression material and impression technique may also influence the final result.

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