Manufacturing metallic materials
Most metals are found as ore (a rock with metal compounds). The path is:
- Mining the ore, then crushing and concentrating it.
- Extraction: iron ore is heated with coke and limestone in a blast furnace to make pig iron. Aluminium is made by electrolysis, which needs a lot of electricity.
- Refining: in a steel converter, air or oxygen burns out extra carbon and impurities from the pig iron, and alloy metals are added to make steel.
- Casting: the liquid metal is poured into a mould and cools to a solid ingot or a part. Ingots are then rolled, forged or drawn into sheets, bars and wires.
Making a metal from recycled scrap needs far less energy than making it from ore.
Manufacturing ceramic materials
Ceramics start as fine powder (clay, silica, alumina).
- Prepare: grind and mix powder with water or a binder.
- Shape: press it in a die, pour a thin clay liquid into a mould (slip casting), or push it through a nozzle (extrusion).
- Dry slowly so it does not crack.
- Fire (sinter) at about 1000 to 1700 °C. The grains fuse at their touching points and the part becomes hard and shrinks a little.
- Glaze (optional): a glass-like coating for a smooth, waterproof face.
Glass is made by melting sand, soda and lime at about 1500 °C and shaping it while it is thick and glowing. Cement is made by heating limestone and clay in a kiln, then grinding the result with a little gypsum.
Manufacturing polymer materials
Polymers are built from small molecules called monomers, mostly made from oil or gas.
- Addition polymerisation: monomers with a double bond simply add one after another. Ethene becomes polyethylene.
- Condensation polymerisation: monomers join and lose a small molecule such as water each time. Nylon and polyester are made this way.
The result is a pile of long chains. Additives are mixed in: plasticisers make it soft, dyes give colour, fillers cut cost. Then the polymer is shaped by injection moulding (melt pushed into a mould), extrusion (pipes and sheets), or blow moulding (bottles). Thermoplastics can be melted again; thermosets set for good.
Manufacturing composite materials
A composite has a reinforcement (strong fibres such as glass, carbon or steel rods) in a matrix (resin, metal or ceramic) that holds the fibres and passes the load to them.
- Hand lay-up: lay fibre mats in a mould, brush in resin, let it cure. Used for boat hulls.
- Filament winding: resin-wet fibre is wound round a spinning shape. Used for tanks and pipes.
- Prepreg and autoclave: fibre pre-soaked in resin is stacked and baked under pressure. Used for aircraft parts.
- Pultrusion: fibre is pulled through a resin bath and a heated die to make long rods and beams.
- Concrete is also a composite: cement paste with sand and gravel, with steel rods inside.
Fibre fraction = fibre volume ÷ total volume. More fibres usually mean more strength, up to a limit.
Try it: a rolled newspaper is weak, but many sheets glued in layers make a stiff board. That is the composite idea.
Key formulas and definitions
- Mass = density × volume (casting: mass of the part)
- Fe₂O₃ + carbon monoxide → iron: 160 g of Fe₂O₃ gives 112 g of Fe
- Chain mass = monomer mass × number of units
- Fibre fraction = fibre volume ÷ total volume
- Fired shrinkage % = (size before − size after) ÷ size before × 100
Worked examples
1. A mould of 10 cm × 10 cm × 5 cm is filled with iron (density 7.8 g/cm³). Find the mass of the casting.
Volume = 10 × 10 × 5 = 500 cm³. Mass = 7.8 × 500 = 3900 g = 3.9 kg.
2. How much iron can be got from 160 kg of pure Fe₂O₃? (Fe = 56, O = 16)
Fe₂O₃ = 2×56 + 3×16 = 160. It holds 2 × 56 = 112 of iron. So 160 kg gives 112 kg of iron.
3. A polyethylene chain has 1000 ethene units (28 u each). Find the chain mass.
1000 × 28 = 28 000 u.
4. A pressed ceramic block is 100 mm long and becomes 88 mm after firing. Find the shrinkage %.
(100 − 88) ÷ 100 × 100 = 12%. So the mould must be made about 12% bigger.
5. A composite of 100 cm³ contains 40 cm³ of fibre. Find the fibre fraction.
40 ÷ 100 = 0.4, that is 40%.
Common mistakes
- Thinking ceramics are melted and poured like metals. They are pressed from powder and fired.
- Forgetting shrinkage in ceramics. The part gets smaller during drying and firing.
- Mixing up the roles in a composite: fibres carry the load; the matrix holds them and spreads the load.
- Thinking all plastics can be re-melted. Thermosets cannot.