“Lost wax” precision casting is an ancient technique invented more than 3000 years ago in Mesopotamia, which was applied only to jewellery, dental prostheses, artistic items, etc.
It arose at an industrial level after the Second World War thanks to the boom in armaments, transport machinery and aerospace.
The development of refractory materials and the increasingly widespread use of expensive alloys (based on nickel) and/or those that are difficult to machine, saw the expansion of the lost-wax technique in the industrial world.
As it can produce parts of complex shapes and astonishingly-precise dimensions, this type of casting adapts well to these new requirements.
Since then, it has continued to progress and develop in numerous sectors of activity.
The principle consists of using metal injection moulds to produce an exact replica (known as the model) of the final part, made of a material that can be melted (wax). Each injected model will ultimately provide a metal part. So if the requirement is for 500 parts, 500 wax models must be injected.
These models are then glued onto bars, also made of wax, which constitute assemblies known as clusters.
These clusters are then coated with layers of ceramic by specific soaking operations in baths of ceramics and are sprinkled with refractory sand.
About 6 to 10 layers are thus built up, which forms a shell of 5 to 10 mm thick, which completely coats all of the parts in the wax cluster.
After a period of drying, this cluster is then heated, which melts the wax, providing a hollow ceramic mould into which the molten metal will then be poured.
This refractory mould is then placed in a furnace at about 1000°C for 1 to 2 hours, which both fires the mould, eliminates any wax residue and heats the mould to help the flow of molten metal.
At the same time, the metal is melted in induction furnaces, between 1500°C and 1600°C for alloys of iron, cobalt or nickel.
When the mould reaches the right temperature and the molten metal is ready, it is poured into the cluster by gravity and it is then left to cool.
The shell is then eliminated by mechanical or chemical action, and the parts are detached from the cluster by cutting.
They are then finished by operations involving grinding, sandblasting, thermal treatment,… as well as visual, dimensional and non-destructive checks (x-ray, dye-penetrant testing).
This technique is mainly used for producing parts of complex shapes, requiring an excellent surface condition and very accurate dimensional precision. It is often an attractive alternative to the technique of machining from a single piece or assembly by welding and is sometimes necessary for parts having internal cavities, thin walls, recesses and reliefs, reinforcements, etc. It is particularly effective for making steel parts from 1 g to more than 50 kg.
All grades of steel and alloys of copper, nickel and cobalt can be cast using this technique. Parts made of titanium or super alloys can also be made using this technique, but with the use of special techniques of casting under vacuum or centrifuges.