Selecting Materials for Manufacture
The Selection Problem
Choosing the right material means balancing five competing factors: functional requirements (the product must work), cost (affordable), availability (obtainable in the right form), aesthetics (appropriate appearance), and sustainability (environmentally responsible). The specification covers all five, and exam questions regularly ask students to justify a material choice against two or more of them.
Properties vs Requirements
The primary criterion is always whether the material's properties meet the product's functional requirements. A functional requirement is a specific performance demand: the bracket must support 500 N without yielding; the casing must resist impact. Engineers map requirements onto properties: a spring requires elasticity; a cutting tool requires hardness; a body panel requires malleability. A material is eliminated if any critical property falls short, regardless of its other advantages.
Engineers may use property charts (such as Ashby charts) to shortlist candidates — a shaft that must be stiff and lightweight might shortlist carbon fibre, titanium, and aluminium alloy before cost and availability narrow the choice. All the properties covered earlier — strength, elasticity, ductility, hardness, malleability, machinability — feed into this stage.
Material selection always starts with functional requirements — a material that does not meet the minimum performance demands cannot be chosen, regardless of cost or sustainability advantages. Properties are the gating criterion.
Cost
Cost has two specification components: material cost (price per kilogram or standard section) and manufacturing cost (tooling, energy, labour and machine time to process it). They can pull in opposite directions: CRP has a very high material cost but its low density means less material per component, yet manufacturing requires autoclaves, adding significant processing cost. Aluminium has moderate material cost but high machinability, giving low manufacturing cost for CNC-milled parts. A unit cost analysis considers both together to give the true cost per component.
For a one-off prototype, a higher material cost per kilogram may be acceptable if the material is easy to hand-process, minimising manufacturing cost. For mass production of 100,000 units, even a small reduction in material cost per kilogram saves a large total sum, justifying precision tooling investment. The tooling cost for injection moulding or die casting is only recovered at high volume — for one-offs, machining a polymer block (more material waste, but no tooling) is usually cheaper.
Availability
Even the ideal material on paper is useless if it cannot be obtained when needed. Availability covers three aspects: stock form (is the material supplied as bar, sheet, tube, or extrusion in the needed profile?), lead time (how quickly can it be delivered?), and supply certainty (single-supplier risk?). Low carbon steel, aluminium alloy, ABS, and acrylic are widely stocked and delivered within days. Specialist materials such as Nitinol SMA wire or engineering ceramics can have lead times of weeks from only a few global suppliers, making supply certainty a key concern at production scale.
Aesthetics
Aesthetics — the visual and tactile qualities of a material — matter when the material is visible in the finished product. Colour, texture, translucency and achievable surface finish are all aesthetic properties. PMMA/acrylic is chosen for display cases for its optical clarity and mirror-polish capability. Stainless steel conveys quality and hygiene in kitchen and medical products. Brass is chosen for fittings and instruments partly for its warm golden colour. Between two functionally equivalent materials, aesthetics can be the deciding factor, especially for consumer products where appearance drives purchasing.
The five material selection criteria are: properties versus requirements, cost (material and manufacturing), availability, aesthetics, and sustainability. In most engineering scenarios more than one criterion is used simultaneously to shortlist and then select the final material.
Sustainability
Sustainability considers the full material lifecycle: extraction (mining energy and habitat disruption), processing (smelting or polymerisation emissions), product life (durability, repairability), and end-of-life (recycling potential, biodegradability). Key examples: primary aluminium has very high embodied energy, but recycled aluminium uses only ~5% of that — closed-loop recycling is essential. PLA is made from renewable plant starch and is industrially compostable, unlike petroleum-derived ABS. Steel has the highest global recycling rate of any engineering material and retains properties through multiple cycles. Sustainability requirements are now embedded in procurement policies and environmental legislation.
Balancing the Criteria
Real decisions balance all five criteria. A product might use a slightly less strong material if it is cheaper and more available. A consumer product might pay for a better-looking material if aesthetics justify a higher price. Sustainability is now weighted more heavily as companies must meet environmental procurement standards. The exam tests this balancing directly: 'evaluate the selection of a material' questions require at least two criteria and a reasoned judgement, not a list of properties.
Use this four-step structure for evaluation answers. (1) State the functional requirement: 'the housing must resist impact outdoors.' (2) Name the property and a material that meets it: 'polycarbonate has very high impact strength and can be UV-stabilised.' (3) Evaluate cost: 'polycarbonate is more expensive than HIPS but its impact resistance avoids product recalls.' (4) Consider sustainability: 'polycarbonate is not biodegradable, but its long service life lowers replacement frequency and overall environmental impact.' End with a clear justified conclusion. This structure addresses multiple criteria and reaches a recommendation — both needed for Mastering marks.