How do you choose the right material for a product when properties, cost and processing all pull in different directions?
The classification of materials into papers and boards, timbers, metals, polymers, composites and technical textiles, and the physical and mechanical properties that decide which material suits a given application.
A focused answer to AQA A-Level Design and Technology Product Design 3.1.1, covering how materials are classified into the main families and the physical and mechanical properties used to select a material for a product.
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What this dot point is asking
AQA wants you to classify materials into the main families, know representative examples of each, and explain the physical and mechanical properties that govern whether a material is suitable for a particular product and process.
Classifying materials
Materials fall into recognised families, and you should be able to give examples of each.
- Papers and boards: layout paper, cartridge paper, corrugated card, foam board.
- Timbers: natural hardwoods (oak, beech) and softwoods (pine), plus manufactured boards (plywood, MDF, chipboard).
- Metals: ferrous (mild steel, cast iron, which contain iron and rust), non-ferrous (aluminium, copper) and alloys (brass, stainless steel).
- Polymers: thermoforming (acrylic, polypropylene, which can be reheated and reshaped) and thermosetting (epoxy resin, urea formaldehyde, which set permanently).
- Composites and technical textiles: carbon fibre reinforced polymer, glass reinforced plastic, Kevlar and conductive or fire-resistant fabrics.
Physical properties
For example, copper is chosen for wiring because of its high electrical conductivity, and aluminium is used for aircraft because of its low density. Density (mass per unit volume) decides how heavy a part of a given size will be and underlies the important idea of strength-to-weight ratio: a racing frame or an aircraft wants high strength for low mass, which is why low-density materials such as aluminium, titanium and carbon fibre composites dominate there. Thermal conductivity decides whether a material feels warm or cold and whether it conducts or insulates heat, so a saucepan body is a conductor (aluminium) while its handle is an insulator (a thermosetting polymer). Optical properties (transparency, translucency) matter for lenses and packaging, and magnetic behaviour distinguishes most ferrous metals (magnetic) from non-ferrous metals and polymers (not), which is exploited in sorting scrap for recycling.
Mechanical properties
The distinctions within each mechanical property are where marks are won. Strength must be specified by type: tensile (pulling), compressive (squashing) or shear (sliding), because a material strong in one may be weak in another (concrete is strong in compression but weak in tension, which is why it is reinforced with steel). Hardness (resistance to scratching and indentation) is not the same as toughness (resistance to sudden impact and crack propagation): glass is hard but brittle, while many polymers are soft but tough. Stiffness (resistance to bending, governed by the elastic modulus) is separate again from strength: a material can be strong but flexible. Ductility (drawn into wire) and malleability (shaped under compression without cracking) describe how a material can be formed, and fatigue resistance describes whether it survives many repeated load cycles, which is why a component that is strong enough for a single load can still fail after thousands of cycles.
Selecting a material
Material choice is always a balance: a property that suits the function may make the part harder or more expensive to manufacture. You weigh the required properties against the manufacturing process (can it be moulded, machined, joined?), the cost and availability, the aesthetic requirements (colour, finish, feel) and the environmental requirements (recyclability, embodied energy, sourcing). A good answer treats selection as a justified trade-off: it names the property the application demands, identifies a material family that delivers it, then checks that the choice is realistic to manufacture at the required scale and affordable. Carbon fibre may give the best strength-to-weight, but if the product is a budget item made in millions, a moulded polymer that is "good enough" and far cheaper is the better engineering choice. This is why the same function can be met by very different materials depending on cost, volume and context.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20186 marksA designer is selecting a material for the frame of a racing bicycle. Discuss the physical and mechanical properties that matter, and explain why a composite such as carbon fibre reinforced polymer might be chosen over aluminium alloy. [6 marks]Show worked answer →
A Paper 1 extended item assessing AO3. Markers reward properties linked to the application, not a property list. Award marks for naming the relevant properties: low density (physical) for a light frame; high tensile and compressive strength and good stiffness (mechanical) to resist riding loads; fatigue resistance for repeated stress; and toughness so it does not fail brittlely on impact. Award marks for the comparison: carbon fibre reinforced polymer has a very high strength-to-weight ratio and stiffness, and its lay-up can be tailored so fibres run along the main load paths, giving a lighter, stiffer frame than aluminium; aluminium alloy is cheaper, easier to repair and recycle, and tougher in a crash. A top answer judges that for a racing application the weight and stiffness gains justify the higher cost and harder repair of the composite.
AQA 20214 marksExplain the difference between a thermoforming polymer and a thermosetting polymer, and give one product example of each. [4 marks]Show worked answer →
A short-answer item. The common error is to call every plastic a "thermoplastic". Award marks for: a thermoforming (thermoplastic) polymer softens when heated and can be reshaped and recycled repeatedly because heating does not change its chemical structure (example: acrylic in a display stand, or polypropylene in a chair); a thermosetting polymer undergoes a chemical change (cross-linking) when first cured, setting permanently, so it cannot be remelted or reshaped and resists heat (example: epoxy resin in a circuit board, or urea formaldehyde in an electrical fitting). Full marks need the reheatable-versus-permanent distinction plus a valid example each.
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