Specialised moulding and casting processes
Domain 8 - Specialised moulding and casting processes
This domain tests when to use shell, investment, die, centrifugal, CO2, no-bake, lost-foam and permanent-mould processes, and their advantages and limits for ferrous castings.
Processes
- Shell moulding uses resin-coated sand on a heated metal pattern; it gives good finish and dimensional accuracy.
- Investment or lost-wax casting uses a wax pattern, ceramic shell and autoclave dewaxing; it suits complex, thin-walled and high-melting alloys.
- Die casting forces metal into a steel die under pressure; it is mainly for aluminium, zinc and magnesium, not steel.
- Centrifugal casting is used for pipes and cylinder liners.
- CO2 moulding hardens sodium-silicate sand with carbon dioxide gas.
- No-bake moulding uses resin and hardener; lost-foam uses a polystyrene pattern that vaporises.
- Permanent mould casting uses a reusable metal mould, usually for non-ferrous work.
Worked example
For a small, intricate steel component with a 2 mm wall, investment casting is chosen. A wax pattern is injected, assembled on a sprue, dipped in ceramic slurry and stuccoed repeatedly, dewaxed, fired, and poured. The ceramic shell must be strong enough to hold the metal but permeable enough to let gases escape. Shell moulding would be cheaper for simpler shapes, but the thin wall and detail favour investment.
Traps
Do not select die casting for steel; die life and temperatures are impractical. Shell moulding is not the same as shell core making. Centrifugal casting is not ideal for irregular asymmetric shapes. CO2 moulding needs controlled gassing; over-gassing makes the mould friable.