Key Stage 3 design and technology · Make

KS3 design and technology: choosing materials by properties

In Key Stage 3 design and technology, pupils meet a wider and more complex range of materials, components and ingredients than at primary school, and learn to choose between them using their properties. They also study how the parts of a structure carry forces, so that a product both works and lasts. The aim is a reasoned choice: not “I like wood” but “wood is strong enough for the job, easy to work and cheaper than the alternatives”.

Choosing materials by properties, explained

A property is something a material does or is. Pupils learn a working vocabulary of properties: strength (resisting a force without breaking), stiffness (resisting bending), hardness (resisting scratching or denting), toughness (absorbing a knock without cracking), elasticity (springing back), malleability (being shaped by hammering or pressing), density (how heavy it is for its size) and conductivity (how well it carries heat or electricity). Corrosion resistance and the ability to be recycled are also considered. Properties are compared, not just listed: steel is stiffer than aluminium, but aluminium is lighter.

The curriculum asks for a wider, more complex range of materials, components and ingredients, and schools choose which to cover. Typical groups are woods (softwoods such as pine, hardwoods such as oak, and manufactured boards such as plywood), metals (ferrous metals such as steel, which contain iron and rust, and non-ferrous metals such as aluminium and copper), polymers (thermoforming plastics that can be softened by heat again and again, and thermosetting plastics that set permanently), and textiles (natural fibres such as cotton and wool, and synthetic fibres such as polyester). Components include fixings, hinges, fasteners and electronic parts. In food, the ingredients are chosen for what they do: eggs set when heated, fats add richness, and flour, when mixed with water and worked, forms gluten.

Choosing is a trade-off. Rarely is one material best at everything, so pupils weigh properties against the product’s needs and against cost, appearance, availability, how it can be made and what happens at the end of its life. Many teachers use a simple table that scores each material against the requirements, with the highest total usually the best choice. The clever part is deciding which requirements matter most. A chair leg needs strength more than low cost, while a disposable cup needs the reverse.

The second objective, understanding and using the performance of structural elements, is about forces. Pupils learn that a material in tension is being pulled, in compression is being squashed, in bending is being curved, in torsion is being twisted and in shear is being slid apart. Good structures use shape as well as material: a triangle cannot change shape without changing the length of a side, so triangulated frames are stiff, and a tube or an I-beam resists bending very well for the amount of material it uses, because most of the material is placed far from the middle, where the bending forces are greatest. Reinforcing a card model with folds or ribs demonstrates this clearly.

Properties can also be changed. Heat treating, laminating thin layers to make plywood, adding a coating or galvanising steel (coating it in zinc to stop rust) all alter what a material does. In textiles, a blend of cotton and polyester gives something that is both comfortable and crease-resistant. Understanding this helps pupils explain not only what to use, but how to adapt it.

Words your child will hear

Property: A characteristic of a material, such as strength, hardness, density or conductivity.
Tension and compression: Tension is a pulling force and compression is a squashing force.
Ferrous metal: A metal that contains iron, such as steel. Most ferrous metals rust.
Thermoforming plastic: A plastic that softens when heated and can be reshaped again and again.
Composite: A material made from two or more materials joined to get the best of each, such as fibreglass or carbon fibre.
Triangulation: Using triangles in a frame to make it rigid.
Corrosion: Slow damage to a metal from a chemical reaction, such as rusting in damp air.

What the National Curriculum says

The programme of study for Key Stage 3 says:

When designing and making, pupils should be taught to select from and use a wider, more complex range of materials, components and ingredients, taking into account their properties
When designing and making, pupils should be taught to understand and use the properties of materials and the performance of structural elements to achieve functioning solutions

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

Try it: choosing a material for a shelf bracket

A pupil is designing a bracket to hold a shelf that will carry paint tins in a garden shed. Which material should they choose, and why?

List what the bracket must do. It must carry a heavy load without bending, cope with damp, and be affordable to make several. These requirements give the properties to look for: strength, stiffness, corrosion resistance and low cost.
Work out what forces act. The shelf’s load pushes down on the end of the bracket. In an L-shaped bracket with a diagonal strut, the strut is squeezed (compression) and the top edge is pulled (tension). The diagonal also makes a triangle, which stops the bracket folding flat.

Compare candidate materials against the requirements, scoring each from 1 (poor) to 3 (good).

Material   Str Stiff Damp Cost Total
Steel       3    3    1    2    9
Aluminium   2    2    3    1    8
Pine        2    2    1    3    8
Acrylic     1    1    3    2    7

Look beyond the totals. Steel scores highest, but its weakness is rust, so it must be painted or galvanised. Because the shed is damp, the pupil makes the damp column count double, which lifts aluminium to 11 against steel on 10 and pine on 9.
```
Steel:     3 + 3 + (1×2) + 2 = 10
Aluminium: 2 + 2 + (3×2) + 1 = 11
Pine:      2 + 2 + (1×2) + 3 = 9
```
Make a reasoned choice and say what remains. Aluminium is lighter and does not rust, but it costs more and is less stiff, so a deeper section or a bracing rib is needed. Steel, coated, is also a sound option if cost matters most.

Aluminium, with a triangulated shape, because it resists damp, is light, and can be made stiff enough by its shape. The pupil should note that coated steel is a close second and that the decision depends on how much weight is given to cost.

Common mistakes and how to help

Saying a material is “strong” when they mean something else

In everyday language, strong covers hard, stiff, tough and heavy. Glass is hard but not tough, while rubber is tough but not stiff.

What helps: Test each property separately: scratch it for hardness, bend it for stiffness, and drop or knock it for toughness. Use the exact words and say what test would show it.

Thinking all plastics are the same, or that all metals rust

Everyday words are broad, so “plastic” covers everything from carrier bags to kettle handles. Only iron-containing metals rust in the usual way. Aluminium forms a thin oxide layer that protects it.

What helps: Sort real objects into groups and give a reason for each, for example “this is a thermoforming plastic because it can be reshaped by heat”.

Believing a solid, thick piece is always stronger than a shaped one

More material feels safer. But an I-beam or a hollow tube resists bending very well for its weight, and a thick but badly shaped piece may fail.

What helps: Try it with paper: a flat sheet bridging two books sags, while the same sheet folded into a corrugated shape or rolled into a tube holds a load.

Choosing the favourite material rather than the best fit

Pupils like working with some materials, such as wood or acrylic, and the choice is made before the requirements are looked at.

What helps: Ask which property the product needs most and which material is best for it. A scored table makes the reasoning visible and open to challenge.

How to help at home

Property hunt. Pick five everyday objects and name the material and the property that makes it suitable. For example, a saucepan handle is plastic or wood because those do not conduct heat well.
Paper structures. Bridge two piles of books with a sheet of paper, then try folding it, rolling it or adding a triangle shape. See which holds the most coins before sagging, and talk about tension and compression.
Cook and compare ingredients. In a supervised kitchen, make the same simple biscuit or pancake recipe with one change, such as leaving out the egg or using less fat. Discuss what that ingredient does for the texture.
Read the label. Check clothing labels for fibre content, such as cotton, wool, polyester and blends. Ask why a waterproof coat and a T-shirt use different fibres.
Score a choice together. Choose a water bottle or lunch box and make a simple scoring table for materials, with agreed weightings. It shows that the best choice depends on what matters most.

Builds on

Key Stage 2: Using tools accurately
Key Stage 2: Choosing materials