Casting methods in depth: from sand mould to investment casting
Why choose one method over another?
You already know sand molds and permanent molds from your everyday work in the foundry. But the industry has several specialized casting methods, which are used when the part places special demands on precision, surface finish, or geometry. Two of the most important are investment casting and permanent mold casting.
§Investment casting – also called lost wax
Investment casting is used when a piece needs very high precision and complicated geometry that is difficult or impossible to make with a regular sand mold. The method is also called 'lost wax' because it is based on a wax model that is ultimately lost in the process.
| Step | What happens |
|---|---|
| 1. Wax model | Melted wax is injected into a precision mould so a wax model of the item is created |
| 2. Run-in system | Wax models are assembled on a feeding system — the channels through which metal must later flow |
| 3. Formwork cladding | The wax cluster is dipped repeatedly in a ceramic slurry until a solid ceramic shell is built up on the outside |
| 4. Wax melting | The shell is heated so the wax melts and runs out — left behind is a hollow ceramic negative of the item |
| 5. Casting | The liquid metal is poured into the hot ceramic shell and fills exactly the space the wax left |
| 6. Completion | After cooling, the ceramic shell is broken off and the parts are cut free from the gating system |
§Die casting: the reusable metal mold
Where the sand mould is discarded after one casting, a crucible is a reusable metal mould. It withstands many castings in a row and is typically used for items that are to be produced in large, identical series — for example in non-ferrous metals with a relatively low melting point.
The advantage is uniformity: the same shape casting after casting gives a very stable result and a good surface without needing to make a new mould each time. In return the actual mould itself is a more expensive investment than a sand mould so the method must be paid back by the number of pieces.
§To choose the right method
- 01Single pieces or small series with complex parts → sand mold or investment casting
- 02Very high precision and finish, price is secondary → investment casting
- 03Large, uniform series in the same alloy → chill casting or pressure casting
- 04Very large or heavy items → sand form (shell moulds and ceramic shells have practical size limits)
As a foundry technician it is your professional judgement — together with drawing order quantity and product requirements — that determines which method actually gives the best result at the right price. There is no single method that is best for everything.
“The method is chosen by the object, not the other way around. A good foundry masters several techniques and knows when each of them pays off.”