The big idea: Properties tell you which materials could work. Cost, availability and sustainability decide which one you actually use.
All three are bigger than the sentence students write, and they rarely point the same way.
Each factor expanded from the naive version into the questions that really decide a choice — then the conflict between them.
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Cost — what students write
- "Aluminium is more expensive than steel."
- A price per kilogram
- Ignores what happens after buying it
Cost — what decides a choice
- Cost per part: a lighter part uses less material
- Processing: what it takes to cut, form, join and finish it
- Waste: how much of the bought stock ends up in the part
- Cost over the life: a coating that fails at three years is bought twice
| Factor | The questions that matter | On the stool |
|---|---|---|
| Availability | In the section and grade you need; lead time; minimum order; a second source | 25 mm stainless tube is stocked locally; 22 mm is a six-week special order |
| Sustainability | Energy to extract and process; transport; how long the part lasts; what really happens at the end | Recycled aluminium halves the extraction energy but adds six weeks |
| Cost | Per part, processing, waste, and over the whole life | Powder coating is cheap now and repainted every three years |
A long life is a sustainability decision: A stool that lasts fifteen years in a slightly worse material beats one that lasts four in a better one.
Durability, repairability and replaceable parts belong in a sustainability answer just as much as recycled content does — and they are the half students leave out.
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Resolving a conflict, in an order you can defend
Essentials are not negotiable
A material that fails corrosion resistance in a wet lab is out, whatever it costs. Cost never buys back an essential property.
Cost over the LIFE, not at purchase
A school buying 200 stools cares about the fifteen-year cost. A coating repainted twice is more expensive than stainless that never is.
Availability limits what is real
A material you cannot get in the section you need, in time, is not a choice. Design round the stocked section rather than specifying a special.
State the trade-off you accepted
"Recycled aluminium was rejected for a six-week lead time; virgin aluminium was used and the frame designed to be re-anodised rather than replaced."
Design round what is stocked: Standard sections, standard sheet thicknesses and standard fixings are cheaper, faster and available from more than one supplier.
Specifying 22 mm tube because it looked better in CAD can add six weeks and a minimum order to a school project.
How this is tested — weighing cost, availability and sustainability in a specific context. It comes up two ways:
Paper 1 — multiple choice
- Identify which factor a stated constraint belongs to.
- Pick the material choice that follows from stated factors.
Paper 2 — analysing a product
- Identify appropriate materials for a named product using these factors.
- Explain a trade-off between cost and sustainability.
The trap: Answering sustainability with a recycling symbol. Extraction energy, transport, product life and what really happens at end of life are all part of it — and a longer life is often the biggest lever.
A school must buy 200 lab stools. Evaluate coated mild steel, virgin aluminium and recycled aluminium for the frame, using cost, availability and sustainability.
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