The key physical properties are density, thermal conductivity, and melting point. Designers compare each property with the product specification because the best material must perform safely and efficiently in its intended environment.
Density is mass per unit volume:
It is commonly measured in . Low-density materials reduce product mass, which is important in aircraft, vehicles, portable products, and sports equipment. However, low density does not automatically mean that a material is weak; aluminium alloys and composites can have high strength-to-weight ratios.
Thermal conductivity describes how readily thermal energy passes through a material, usually measured in . High conductivity is desirable for heat sinks and cookware, while low conductivity is required for insulation and heat-resistant handles.
Melting point is the temperature at which a material changes from solid to liquid. A material used near a heat source must have a melting point sufficiently above its operating temperature, although some polymers soften across a temperature range rather than having one sharply defined melting point.
| Property | When a high value is useful | When a low value is useful |
|---|---|---|
| Density | Counterweights, stability, vibration reduction | Aircraft, bicycles, portable products |
| Thermal conductivity | Saucepans, radiators, electronic heat sinks | Building insulation, saucepan handles, protective clothing |
| Melting point | Engine parts, ovens, manufacturing tools | Low-temperature casting and fusible safety components |
Material selection requires balancing these properties. For example, copper transfers heat effectively but has a higher density than aluminium. Aluminium may therefore be selected for a lightweight heat sink, even though copper has greater thermal conductivity.
A common misconception is that one favourable property makes a material suitable. In an IB Design Technology response, justify the selection by linking each relevant property to the product's function, operating conditions, safety requirements, and specification. Include units or comparative data when provided, and explain trade-offs rather than merely naming a material.