The main material families
| Family | Examples | Typical properties |
|---|---|---|
| Papers and boards | copier paper, card, corrugated board | light, cheap, easy to cut, fold and print; weak when wet |
| Timbers | hardwoods (oak, teak, sheesham), softwoods (pine), manufactured boards (MDF, plywood) | strong for their weight, good insulators, attractive grain; can rot or warp |
| Metals | ferrous (contain iron: steel, cast iron); non-ferrous (aluminium, copper); alloys (brass, stainless steel) | strong, good conductors, malleable and ductile; most ferrous metals rust |
| Polymers | thermoplastics (acrylic, PET, nylon) can be reheated and reshaped; thermosets (melamine, epoxy) set hard for ever | light, waterproof, insulators, easily moulded into complex shapes |
| Textiles | natural fibres (cotton, wool, silk), synthetic fibres (polyester, nylon), blends | flexible, soft, can be woven or knitted; absorbency and warmth vary |
Newer smart and composite materials (like carbon fibre) combine properties, but they build on these families.
Physical properties
Physical properties describe what the material is like, without forcing it.
- Density: mass per unit volume (density = mass ÷ volume). Steel ≈ 7.8 g/cm³, aluminium ≈ 2.7, acrylic ≈ 1.2, pine ≈ 0.5. Low density matters for bikes and aircraft.
- Thermal conductivity: how well heat passes through. Metals are good conductors (pan bases); wood and plastics are insulators (handles).
- Electrical conductivity: copper wires conduct; PVC covering insulates.
- Absorbency: how much liquid it soaks up. Cotton towels absorb; polyester raincoats repel.
- Fusibility: how easily it melts and joins when heated (metals for casting, thermoplastics for moulding).
- Corrosion resistance: how well it resists rust and decay. Aluminium and stainless steel resist; mild steel rusts unless painted.
Working (mechanical) properties
Working properties describe how a material behaves when a force acts on it or when we shape it.
- Strength: resists breaking under a force (pulling = tensile, squashing = compressive).
- Hardness: resists scratches, wear and dents (steel files, ceramic tiles).
- Toughness: absorbs a sudden blow without cracking (a nylon hammer head). Opposite: brittle (glass, cast iron).
- Elasticity: returns to its shape after the force is removed (rubber bands, springs).
- Plasticity: changes shape permanently without breaking (clay, copper being bent).
- Malleability: can be hammered or rolled into sheets without cracking (aluminium foil).
- Ductility: can be stretched into wire (copper wire).
Hard and tough are not the same: glass is hard but brittle; rubber is tough but soft.
Sources, origins and stock forms
Sources: paper and board come from wood pulp (trees) or recycled paper; timber from forests; metals from ores dug out of the ground and then smelted; most polymers from crude oil (some from plants, like PLA from corn starch); textiles from plants (cotton), animals (wool, silk) or crude oil (polyester).
Stock forms are the standard shapes and sizes materials are sold in, so makers can buy and cut them easily:
- Paper: A-sizes (A4 = 210 × 297 mm), card by weight in gsm (grams per square metre).
- Timber: planks, boards, strips, dowels; manufactured boards in sheets (e.g. 2440 × 1220 mm).
- Metals: sheet, plate, bar, rod, tube, angle.
- Polymers: sheets, rods, tubes, granules and powder for moulding, films.
- Textiles: rolls (bolts) of fabric by width; yarn on cones.
Think about the environment: forests can be managed and replanted (look for FSC labels), metals and many polymers can be recycled, and using less material saves energy.
Choosing the right material
Designers ask: What must the product do? Then they match properties to that need. A useful checklist is "FACE + SCE":
- Function: which properties does it need (strong, light, waterproof, insulating)?
- Aesthetics: colour, texture, finish.
- Cost: price of material and of making it.
- Environment: renewable? recyclable? energy to make?
- Sourcing and availability, Cultural and social factors, Ethics (fair work, safe processes).
Example: a child's toy must be tough, non-toxic, smooth (no splinters) and bright, so beech timber with a safe finish or ABS plastic are good choices.
Materials can also be improved while working: timber is sealed against water, steel is painted or galvanised against rust, and fabric can be treated to resist stains or fire.
Key formulas and definitions
- Density = mass ÷ volume (g/cm³ or kg/m³)
- 1 g/cm³ = 1000 kg/m³
- Physical properties: density, conductivity, absorbency, fusibility, corrosion resistance
- Working properties: strength, hardness, toughness, elasticity, plasticity, malleability, ductility
- Ferrous metal = contains iron; non-ferrous = no iron; alloy = mixture of metals
- Thermoplastic = can be reshaped with heat; thermoset = sets permanently
Worked examples
1. A block measures 2 cm × 5 cm × 10 cm and has a mass of 270 g. Find its density and suggest the metal.
Volume = 2 × 5 × 10 = 100 cm³. Density = 270 ÷ 100 = 2.7 g/cm³. This matches aluminium.
2. Why is a saucepan base made of steel or aluminium but the handle of wood or thermosetting plastic?
The base must conduct heat well to cook food, so a metal is used. The handle must not get hot or melt, so a heat insulator that does not soften (wood or a thermoset) is used.
3. Which is the better choice for an outdoor bench in a rainy city: untreated pine or recycled HDPE plastic? Explain.
Recycled HDPE: it does not absorb water, rot or need painting, and it reuses waste. Untreated pine is cheaper but would absorb water and rot unless treated.
4. How many A4 sheets (210 × 297 mm) can be cut from a 1 m² sheet of card, ignoring waste?
Area of A4 = 0.210 × 0.297 ≈ 0.0624 m². 1 ÷ 0.0624 ≈ 16. A0 is 1 m², and it halves 4 times to A4, so exactly 16.
Common mistakes
- Mixing up hard and tough. Hard resists scratches; tough resists cracking from a blow.
- Saying all metals are magnetic or all rust. Only some ferrous metals do; aluminium and copper do not rust like iron.
- Calling every plastic recyclable by melting. Thermosets cannot be remelted.
- Choosing a material only on one property. Cost, looks, availability and environment matter too.