Other processes: lost-wax, lost-foam, shell moulding and centrifugal casting

Lost-wax

The lost-wax casting (or investment casting) process involves the use of a wax or organic resin model, around which a suitably thick layer of refractory material is prepared (by repeated immersion of the model in an aqueous solution containing refractory particles). After initial drying, heating at a moderate temperature causes the material to melt and flow out, constituting the model. The refractory shell thus obtained is sintered and can therefore be filled with the liquid alloy. Once solidification is complete, the shell is destroyed and the casting is obtained.

The process is used to develop medium-small sized precision components with high added value. A typical example would be parts for aircraft turbines made of iron-, nickel- or cobalt-based superalloys using investment casting.

Shell moulding

“Shell moulding” casting uses silica sands, the grains of which are pre-coated with organic resins, and then deposited on metal semi-models. The semi-models are heated with burners or by passing a current through them (Joule effect). This has a cross-linking effect on the organic resins surrounding the sand grains close to the model, which leads to the formation of a shell. The shells are assembled, thus obtaining the mould for the casting. 

This technique is commonly used in foundries and allows the obtainment of highly complex-shaped and very thin castings weighing more or less 20 kg, in very large series. Applications of this technology include cylinder heads, connecting rods, transmission components. Shell moulding is also used to produce high-precision cores.

Centrifugal casting

Centrifugal casting is a foundry process used to create cylindrical or tubular shaped castings. The liquid alloy is poured into a cylindrical, horizontally positioned die that can rotate on its own axis (the operating configuration can, however, be vertical). The centrifugal force generated thus pushes and holds the alloy in contact with the walls of the die, inducing its solidification.  

Use of this technique is documented for the production of axially symmetrical components, for example cast-iron cylinder liners and tubular components for water or gas pipes.

Lost foam

The lost-foam process is similar to lost-wax casting in many ways. In this case too, the model, with sprues, is typically made of polystyrene or poly-methyl-methacrylate, and is disposable. It is coated with a refractory paint and then placed in a container in which loose sand is poured. The sand, which surrounds the model, effectively “generates” the mould; the liquid alloy is poured over the sprue and gradually decomposes the model (the resulting gases escape, exploiting the permeability of the refractory paint and the sand), thus taking its place. When solidification is complete, the casting is easily extracted from the sand.

The process, used in past years to produce relatively large series, has recently been abandoned, not least because of the complexity of the procedure.

Its use is nonetheless documented for complex mould castings, especially in the automotive industry, using not only cast iron but also aluminium, steel, nickel and copper alloys.

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