Common Defect in Lost Wax Casting

The lost wax casting process is widely used as it offers asymmetrical casting with very fine details to be manufactured relatively inexpensively. The process involves producing a metal casting using a refractory mould made from a wax replica pattern. However the process might need to be modified to account for different alloys being used. The method of the alloy melting also depends on the alloy used since the temperatures required are higher than the ones which can be achieved using a torch, or the alloy can chemically react with gases. In order to achieve this accuracy materials used in the process must be chosen as to compensate for the dimensional changes including setting shrinkage of the casting, investment shrinkage/expansion, effect of the confinement induced by the casting ring, temperature induced dimensional changes etc.

The objective of the casting is to provide as accurate duplication of missing teeth as possible. However, the tolerance limits are not set exactly but generally the accuracy should be better than en be detected by the eye or conventional methods. Poor fit / accuracy of the casting could lead to fail of the restoration and lead to further dental problems. Defects in casting can be eliminated or minimised by strict adherence to prescribed procedures.

Generally speaking, the casting defect for investment casting could generally be divided into the followings:

1. Poor surface finish

One of the requirements on the castings is that its surface should be as accurate reproduction of the wax pattern surface as possible. If that is not the case additional operations are required (additional finishing and polishing). The defects under this category can be further divided in excessive surface roughness, unexpected surface irregularities and inappropriate surface colour. Surface roughness can be described as finely spaced surface imperfections, while the irregularities relate more to isolated imperfections (such as nodules). It should be accepted that the surface roughness of the casting is greater than the roughness of the wax pattern it is made of. This can be explained by a particle size of the investment which is such that it cannot reproduce the wax pattern in fine enough details.

2. Distortion

In most cases distortion of the casting can be attributed to distortion of the wax pattern. The wax pattern distortion can be minimised by careful and proper manipulation of the wax and handling of the pattern. In addition, some of distortion of the wax pattern can be caused by hardening of the investment around it, whereby setting and hygroscopic expansion could lead to uneven deformation of the walls of the wax pattern. This depends, on the other hand, on the type of wax, thickness and configuration of the pattern.

3. Incomplete Casting

If the molten alloy is prevented from fully or partially filling the mould and incomplete or even no casting at all can result. The two most obvious reasons for this are insufficient venting of the mould and high viscosity of the.

If the air in the mould could not be vented quickly once the molten allow is poured into the mould, its back pressure will prevent compete filling of the mould by the molten alloy. This point in direction of either insufficiently high casting pressure, or that it has not been applied for a sufficiently long period of time. During the early stages of the casting the metal is still quite soft, but start to solidify (and thicken) quite quickly. High casting pressure should be maintained sufficiently long after onset of solidification.

If the elimination of the wax patter was not fully complete, the pores of the investment might become filled with the combustion products thus preventing venting of the air which can cause similar issue to the above.

4. Porosity

Porosity of the casting can be spread within the casting and on its surface. The surface porosity adds to surface roughness, but can also be a sing of the internal porosity. The internal porosity could weaken the casting, may cause discoloration if spread to the surface and in extreme cases could lead to a leakage.