Lost Wax Casting and Its Common Defects



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. The steps involved in the process or the lost wax casting are:

1- Create a wax pattern
2- Sprue the wax pattern
3- Invest the wax pattern
4- Eliminate the wax pattern by burning it (inside the furnace or in hot water). This will create a mould.
5- Force molten metal into the mould - casting.
6- Clean the cast.
7- Remove sprue from the cast
8- Finish and polish the casting on the die.


Common Defect in Lost Wax Casting

A general description of the process is as given above, however it might need to be modified to account for different alloys being used. Cobalt alloys have melting temperatures between 1250 and 1450 ℃, which is higher than the investment made of gypsum decomposition temperature. Because of that, the gypsum investment must be replaced by phosphate or silica bonded type material. 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. Accuracy of this process is targeted to about 0.1%. 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:
- Distortion,
- Poor surface finish,
- fins / spines
- nodules
- veins and ridges
- rough surface
- Porosity,
- Voids,
- Incomplete casting.

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.

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.

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.

Main causes of the porosity of alloy castings are:

1) Solidification defects
2) Trapped gases
3) Residual air

1 Solidification defects

Solidification defects cold lead to two different manifestations of porosity; localised shrinkage porosity and Microporosity.

Localised shrinkage porosity is caused by insufficient feeding of the alloy during solidification. As the alloy solidify, it also shrinks by over 1%, and a sufficient supply of molten alloy is required during this phase to counteract reduction in the volume caused by the shrinkage. If the sprue is not properly designed and implemented then it may solidify before the feeding is complete thus preventing a continuous supply of molten alloy. This type of defect usually occurs close to the sprue-casting junction. Microporosity is also caused by solidification shrinkage, but generally happens in fine grain alloys when the solidification is too rapid for the microvoids to segregate. This in turn is caused the mould or casting temperature being too low.

2 Trapped gases

Many metals dissolve or occlude gases when they are molten. On solidification, these gases are forced out of the casting causing what is usually called pinhole porosity. These voids are rather small. Larger voids could be caused by the same mechanism, but could also be caused by the gases mechanically trapped within the molten alloy during the casting procedure.

3 Residual air

These voids occur on the inner surface of the casting and are caused by entrapped air which cannot escape through the investment. This can be prevented by adequate temperature during the casting, sufficiently high pressure and correct liquid – powder ratio.

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.

Common Causes of Incomplete Casting

High heating rates

Adopting too high heating rates for the could lead to occurrence of fins (or spines) in the casting. The mechanism for this is as follows:
a) If the investment is subject to too high heating rate the outside layer becomes hot faster than the inner, and the temperature difference between the outside layer and the centre of the investment is increased.
b) Consequently, the outside layer tends to expand more than the inner parts. However, the outside layer is held back by the inner, cooler part.
c) Because of that the outside layer is subject to compressive stresses, while the inner part is subject to tensile stresses.
d) Since the investment is a brittle material, it tends to crack under tensile stresses. In this case, the most typical cracks are radial, starting from the interior of the investment and spreading outwards.
e) During casting, these cracks are filed by the casting alloy, manifesting as fins or spines.

Low temperature

Too low a temperature could cause incomplete removal of the wax. Gases, formed when the hot alloy comes in contact the residues could cause porosity or voids in the casting. Too low a temperature can be caused by too short a heating time or if insufficient air is available in the furnace.

Water film

Wax is a hydrophobic material (i.e. it is repellent to water). If the investment is not in close contact with the wax pattern a water film might be formed over the surface. This water might cause occurrence of small veins and ridges on the surface of the casting. Loss of contact between the investment and the wax pattern might be caused if the pattern has moved slightly, if it has been subjected to vibrations or if the painting has not been properly applied.

Air bubbles

Occasionally, small air bubbles can become attached to the pattern during or following the investing procedure. During the casting, the bubble is filled with the casting material (alloy) and is manifested as a nodule. Occurrence of the air bubbles can be prevented if a vacuum investing procedure is adopted. Some other measures can be adopted to prevent this happening, too, such as use of a mechanical mixer with vibrations or application of a wetting agent in a thin layer.

Liquid – powder ratio

Both too high and too low amount of liquid could lead to a rough surface of the casting.

Prolonged heating

Prolonged heating at too high temperatures could lead to a disintegration of the investment with a consequence of rough mould walls. In addition, products of the disintegration could contaminate the alloy causing surface defects.

Alloy temperature

If the alloy temperature is too high it can attack the surface of the investment similar to the case of the prolonged heating. As a consequence, a similar surface roughness can occur.





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