Describe the casting and moulding processes used to form metal components, including sand casting, die casting and investment casting, and explain their applications, advantages and limitations

What this dot point is asking

Casting forms metal by pouring it molten into a mould, and it is the way complex shapes that would be hard to machine or fabricate are made. NESA expects you to describe the main casting processes, sand casting, die casting and investment casting, and to explain where each is used along with its advantages and limitations. This is core process knowledge for the focus area, examined in the written paper and useful for understanding how cast components in your project are produced.

How casting works

Every casting process follows the same idea. Metal is melted in a furnace, poured into a mould cavity that is the shape of the part, and allowed to solidify and cool before the mould is opened or broken away. The finished casting then usually needs cleaning and machining of critical surfaces. The differences between processes are in how the mould is made, whether it is reused, and how accurate and smooth the result is.

Sand casting

In sand casting, a pattern of the part is packed in special moulding sand to form a cavity, the pattern is removed, and molten metal is poured in. The sand mould is broken away to release the casting, so each mould is used once.

• Advantages: cheap tooling, suits almost any metal, makes very large parts, and is economical for one-off and small runs.
• Limitations: rough surface finish, lower dimensional accuracy, and the need for generous machining allowances.

It is the process behind machine bases, manhole covers and large or low-volume castings.

Die casting

In die casting, molten non-ferrous metal is forced under pressure into a reusable steel mould called a die. The die opens to eject the part and is used again thousands of times.

• Advantages: fast, highly accurate, excellent surface finish, thin walls and fine detail, and very economical at high volume.
• Limitations: high tooling cost, so it is only worthwhile for large quantities, and it is mostly limited to lower-melting non-ferrous metals such as aluminium and zinc.

It produces items like engine and appliance components in their thousands.

Investment casting

Investment casting, also called lost-wax casting, uses an expendable wax pattern that is coated in a ceramic shell. The wax is melted out, leaving a cavity into which metal is poured, and the shell is broken away.

• Advantages: very high accuracy, fine detail and smooth finish, suits intricate shapes and a wide range of metals, with little machining needed.
• Limitations: slower and more expensive per part, suited to detailed components rather than large simple ones.

It makes turbine blades, jewellery and precision engineering parts.

Casting defects

Castings can fail in characteristic ways: porosity from trapped gas, shrinkage cavities as metal cools, cold shuts where two streams fail to fuse, and inclusions of slag or sand. Good mould design, venting and pouring control reduce these, and understanding them explains why castings are inspected before use.

Choosing a casting process

Select by quantity, accuracy and cost. Use sand casting for one-off, large or low-volume parts where finish is not critical and tooling must be cheap. Use die casting for high-volume non-ferrous parts that need accuracy and fine detail and can pay for expensive tooling. Use investment casting for intricate, accurate parts in small to medium numbers. Stating the quantity and accuracy required is the key to justifying the choice.