The lost wax process
Definition
The lost wax process is a moulding process used in atmospheric investment casting. It is therefore also known as the investment casting process.
It is used to produce precise cast objects with high surface accuracy at low production costs. The lost wax process is used in particular for sculptures and cast objects with difficult moulds and high demands on precision and surface quality. This makes the process particularly suitable for objects with complex structures and geometries with many undercuts that are difficult or impossible to realise with other casting processes. During casting, both the model, which is usually made of wax, and the mostly ceramic mould are destroyed. This is why it is also referred to as the "lost mould process" or the "lost pattern process".
History
The lost wax technique is one of the oldest casting processes of all. As early as around 4000 BC, it was mainly used to produce cult objects and objects of artistic casting. But especially in the subsequent Bronze Age from around 2000 BC, numerous objects such as weapons, jewellery elements, bronze doors, everyday objects etc. were produced.
Usage
In addition to our focus on sculptures and statues, the lost wax process is primarily used in the following sectors:
- Art
- Industry and mechanical engineering
- Aerospace and aeronautics
- Dental technology and medicine
Procedure of the lost wax process
If, for example, an artist has delivered a plaster model (original) of his sculpture to the art foundry he trusts, the casting proceeds as follows:
1. Model
The original model - usually made of plaster, wood or 3D printed - forms the basis for the lost wax process.
2. Silicone negative mould
First, the plaster model is embedded about halfway into a blind mould. This also defines the parting surface for the mould halves. The temporary plaster support shell, into which the silicone is poured or applied, is built up on this base. After cooling, the mould can be taken apart and the model removed. The negative silicone mould shows all the fine details of the surface of the original.
3. Wax model
A layer of wax several millimetres thick is now applied to this negative mould. This corresponds to the later wall thickness of the casting. A core (e.g. plaster-fireclay mixture) is poured into the wax sculpture. Once it has solidified, the outer mould can be removed. The elastic gelatine or silicone mould releases the wax model (wax preparation). At this stage, it can be reworked and corrected.
4. Assembly and soldering of the sprue system
Before the entire wax preparation is moulded into a ceramic mass, the sprue and venting channels required for the flowing metal are attached, which serve as the casting system.
5. Dipping, sanding, drying
The assembled wax system is repeatedly dipped into a ceramic slip (moulding slip) and sprinkled with fine sand to form a ceramic shell around the wax. This process is repeated until a sufficiently thick layer is formed to withstand subsequent moulding.
6. Melting out (autoclave)
The ceramic shell is heated to melt out the wax (model) under pressure and create a hollow mould. This is usually done in an autoclave. The melted wax is collected for reuse.
7. Firing the mould
The emptied ceramic mould is heated to harden the ceramic and remove any wax residue. This is also referred to as "annealing the ceramic mould".
8. Casting
Fine casting then takes place in the fired, highly heated mould. After casting, the metal solidifies in the mould and forms the desired cast object.
9. Demoulding
Once the metal has solidified and cooled down sufficiently, the ceramic mould is broken in order to obtain the raw casting.
10. Reworking
After basic processing of the raw casting, such as demoulding and sandblasting, the chaser's work now begins.
First, he carefully cuts off the sprues. Using his arsenal of special tools, he then carries out the reworking. Sculptures cast in several parts are welded together in an inert gas atmosphere. The weld seams are then reworked so that they are practically invisible. At the end of the process, the casting must correspond as closely as possible to the original model, which the chaser constantly refers to for comparison.
11. The final treatment
This is done by the patineur in bronze casting. By brushing, spraying or baking on solutions, he achieves the desired patina through chemical effects on the surface. After a conservation treatment (beeswax, hard wax, etc.), the finished exhibit is now available to the client.
The lost wax casting process described here is based in many respects on our in-house processes and represents the schematic process. Depending on the requirements of the casting order, there are many details and modifications that would go beyond the scope of this article.
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