Casting and moulding processes: HSC Industrial Technology Metal and Engineering
“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”
Casting pours molten metal into a mould cavity, where it solidifies into the shape of the cavity. Sand casting uses a cheap, single-use sand mould around a pattern and suits one-off and large parts. Die casting forces metal into a reusable steel die for fast, accurate, high-volume non-ferrous parts. Investment casting uses an expendable wax pattern in a ceramic shell for highly accurate, detailed parts. Each process trades off setup cost, accuracy, surface finish and quantity.
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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.
A component (a small, detailed aluminium bracket needed in a batch of about 500) must be cast, and the maker must choose and plan the process.
Step 1: Identify quantity, accuracy and metal
Establish the requirements: a medium batch (500), fine detail, a smooth finish and a non-ferrous metal (aluminium).
Step 2: Match the process to the requirements
Compare options: sand casting is too rough and inaccurate, full die-casting tooling is hard to justify at 500, so investment casting (fine detail, smooth finish, suits a range of metals, small to medium runs) best fits, with die casting reconsidered if volume rises.
Step 3: Prepare the pattern and mould
For investment casting, produce wax patterns of the bracket, assemble them on a tree, and coat in a ceramic shell; melt out the wax to leave the cavity.
Step 4: Pour and solidify
Melt the aluminium in a furnace, pour it into the pre-heated shell, and allow it to solidify and cool with controlled venting to limit gas porosity.
Step 5: Finish and inspect
Break away the shell, cut the castings from the tree, clean and machine any critical surfaces, and inspect for defects such as porosity, shrinkage and cold shuts before use.
The economics of casting hinge on the mould. Sand moulds are cheap but single-use, so sand casting suits one-off and large parts. Die-casting dies are expensive but reused thousands of times, so die casting only pays off at high volume, which is why quantity drives the choice of process.
- Ignoring quantity
- Casting choice depends heavily on how many parts you need; do not recommend die casting for a one-off.
- Confusing the processes
- Sand casting uses an expendable sand mould; die casting uses a reusable steel die; investment casting uses an expendable wax pattern. Keep them distinct.
- Forgetting machining allowance
- Sand castings are rough and need machining of critical surfaces; account for this.
- Overlooking defects
- Casting can introduce porosity and shrinkage; mention inspection and good mould design.
Exam-style questions
Questions in the style of NESA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
HSC 2022-style1 markWhich casting process uses an expendable wax pattern coated in a ceramic shell? A. Sand casting B. Die casting C. Investment casting D. ForgingShow worked answer →
The correct answer is C: investment casting. Investment (lost-wax) casting uses an expendable wax pattern that is coated in a ceramic shell; the wax is then melted out and metal poured into the cavity. Sand casting (A) uses an expendable sand mould around a reusable pattern, die casting (B) forces metal into a reusable steel die, and forging (D) shapes solid metal under force and is not a casting process. So C is correct.
HSC 2023-style5 marksCompare sand casting and die casting, and justify which process should be used to manufacture 10,000 aluminium housings that require a smooth finish and fine detail.Show worked answer →
Sand casting packs a pattern in single-use moulding sand, has cheap tooling, suits almost any metal and large or one-off parts, but gives a rough finish and lower accuracy with generous machining allowances. Die casting forces molten non-ferrous metal under pressure into a reusable steel die, giving fast production, high accuracy, excellent surface finish, thin walls and fine detail, but with high tooling cost that only pays off at high volume and is limited to lower-melting metals such as aluminium and zinc. For 10,000 aluminium housings needing a smooth finish and fine detail, die casting is justified: aluminium suits the process, the high die cost is spread across the large run, and the accuracy and finish meet the requirement without much machining. Markers reward the expendable-versus-reusable mould distinction, the cost and finish contrast, and a justified choice of die casting for the quantity and quality stated.