Factors in choosing materials
- Function: what the part must do, and which properties it needs (such as strength, hardness or conductivity).
- Cost: the price of the material and of processing it.
- Availability and forms of supply: metals come as sheet, plate, bar, rod, tube and sections (such as angle and I-beams); polymers come as granules, sheet, rod and film.
- Manufacturing: how easily it can be cut, formed or joined, and whether it suits the scale of production.
- Aesthetics: appearance, colour and finish.
- Environment: whether it is recyclable, how much energy it takes to extract and process, and whether it is finite.
Examples of choices
- Car body panels: pressed steel sheet, because it is strong, cheap and easy to form, or aluminium to save weight.
- Electrical cables: copper for conductivity, with a PVC coating as an insulator.
- Plug casings: urea formaldehyde, a thermoset that is hard, heat resistant and an electrical insulator.
- Aircraft parts: aluminium alloys and carbon fibre composites, for their high strength-to-weight ratio.
Stock sizes and cost
- Buying standard stock sizes reduces cost and waste. Engineers compare cost per kilogram and how much waste a design creates.
Key terms
- Form of supply
- The shape a material is sold in, such as sheet, bar or tube.
- Stock size
- A standard size of material available to buy.
- Strength-to-weight ratio
- How strong a material is compared with its weight.
- Finite resource
- A resource that will run out.
- Insulator
- A material that doesn't conduct electricity or heat well.
- Aesthetics
- How a product looks and feels.