Key Stage 3 design and technology · Technical knowledge

KS3 design and technology: properties of materials

In Key Stage 3, pupils stop describing materials as simply “hard” or “bendy” and start using exact words such as hardness, toughness, elasticity and conductivity. They learn to choose from a wider and more complex range of materials, components and ingredients, and to justify each choice by what the product has to do. Choosing well, and being able to explain why, is the heart of design and technology in Years 7 to 9.

Properties of materials, explained

At primary school a child might say a ruler is “strong”. In Years 7 to 9 that word is no longer precise enough, because a material can be strong in one way and weak in another. Pupils learn to separate the properties of a material, which describe how it behaves: how hard it is to scratch (hardness), how well it absorbs a sudden knock without breaking (toughness), how far it stretches and springs back (elasticity), how heavy it is for its size (density), and whether heat or electricity passes through it easily (conductivity). They also learn the working properties, which describe how the material behaves in the workshop: whether it cuts cleanly, bends without cracking, joins well and takes a finish.

Most schools group materials into families. Timbers include softwoods from conifers, such as pine, hardwoods from broad-leaved trees, such as oak, and manufactured boards, such as plywood and MDF. Metals divide into ferrous metals, which contain iron and can rust, non-ferrous metals such as aluminium and copper, and alloys such as brass, which mix metals to improve their properties. Polymers, the technical word for plastics, include thermoforming types that soften when heated and can be reshaped, such as acrylic, and thermosetting types that set hard for good, such as epoxy resin. Textiles and papers and boards complete the picture. Schools decide which materials to use, and many pupils work with only some of these families in a given year.

Choosing a material is always a trade-off, and this is what the curriculum means by selecting from a wider, more complex range. Pupils weigh up properties against the design specification, along with cost, availability, appearance, how it can be made with the school’s tools, and its environmental impact, including whether it can be reused or recycled. The same thinking applies to components, such as fixings and motors, and to ingredients in food, where a property might be how well flour forms a stretchy dough or how fat changes the texture of pastry.

Material properties are rarely taught from a list to learn by heart. Pupils usually test them: bending strips of different timbers until they snap, scratching samples to compare hardness, hanging weights from strips of plastic. They record results in tables and use them to rank materials. This links to the structural side of the subject, because performance depends on both the material and its shape. A thin strip of steel bends easily, while the same steel folded into an angle is much stiffer. Pupils are expected to use both ideas together to reach a working solution.

Words your child will hear

Property: A feature of a material that describes how it behaves, such as hardness or elasticity.
Hardness: How well a material resists being scratched, dented or worn away.
Toughness: How well a material absorbs a sudden impact without cracking or shattering.
Elasticity: The ability to stretch or bend and then spring back to the original shape.
Malleable: Able to be hammered or pressed into a new shape without cracking, like copper or soft aluminium.
Thermoforming polymer: A plastic that softens when heated and can be reshaped, then hardens again as it cools.
Ferrous metal: A metal that contains iron, such as mild steel. Most ferrous metals are magnetic and will rust.

What the National Curriculum says

The programme of study for Key Stage 3 says:

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
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

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

Try it: choosing a material for a phone stand

A pupil is making a phone stand for a desk. It must hold a 200 g phone steadily, look smart, be made with school tools, and cost little. Which material suits best: pine, MDF, acrylic or aluminium?

Write down what the product must do. It needs to be stiff enough not to bend under the phone, stable, easy to cut and finish in a school workshop, and cheap. Appearance matters, but strength is not the main worry, because 200 g is a small load.
Compare pine. It is a softwood, cheap, light and easy to saw and sand. It can split along the grain if screwed too close to an edge, and the grain shows through paint unless it is prepared well.
Compare MDF. It is a manufactured board made from wood fibres and resin, so it has no grain and can be cut in any direction and painted smoothly. It is cheap, but the dust is harmful, so masks and extraction are needed, it swells if it gets wet, and the edges are rough unless sealed.
Compare acrylic and aluminium. Acrylic is a thermoforming polymer that polishes to a glossy edge and can be heated and bent with a strip heater, which is ideal for an angled stand, but it is brittle and can crack if it is drilled badly or dropped. Aluminium is light, non-ferrous and does not rust, but it costs more and needs more skilled cutting and bending.
Decide against the specification. Acrylic gives the neatest finish and the easiest bend for a stand. MDF would be the cheaper choice if painting is acceptable. The pupil picks one and explains the trade-off.

A strong answer chooses acrylic if appearance and a clean bent shape matter most, or MDF if cost matters most, and names at least one weakness of the choice (acrylic is brittle, MDF swells if wet). The key skill is linking each property to a requirement, not finding a single right answer.

Common mistakes and how to help

Hardwoods are hard and softwoods are soft

The names sound like descriptions of the wood. In fact they describe the type of tree: hardwoods come from broad-leaved trees, softwoods from conifers.

What helps: Balsa is a hardwood and is very light and soft, while some softwoods, such as yew, are quite hard. Look at a few samples together and press a fingernail into each.

Hard, strong and tough all mean the same thing

In everyday speech they overlap. In design they do not: glass is hard but shatters when dropped, so it is not tough.

What helps: Give your child a job and ask which word it needs: a chisel blade needs hardness, a bike helmet shell needs toughness, a climbing rope needs strength when pulled.

All plastics can be melted down and reshaped

Everyday plastic bottles and bags can be recycled, so it seems that every polymer works that way.

What helps: Explain that thermosetting polymers, such as the hard plastic used in some saucepan handles and older electrical fittings, set permanently. Once they have set they cannot be softened again.

All metals rust

Rust is the best-known way metals go wrong, so children assume it affects them all.

What helps: Only metals that contain iron rust. Aluminium and copper corrode in other ways, and aluminium forms a thin oxide skin that protects it. Look for rust on a gate, then compare a drinks can.

How to help at home

Material detectives. Choose one everyday object, such as a chair, a kettle or a bike, and ask your child to name each material and say what property made it a good choice for that part. Ask what else could have been used and why the maker didn’t.
Safe testing at the kitchen table. Compare a wooden spoon, a metal spoon and a plastic spoon: which bends, which feels cold, which scratches easily? Link the cold feeling to conductivity, since metal carries heat away from your hand faster. Keep tests gentle and use nothing sharp.
Read the labels. Clothes labels, packaging and toy markings often give the material. Ask why a coat has a polyester outer layer and a cotton lining, or why a drinks bottle is not made from glass for a school bag.
A fair strength test. Make beams of the same size from different card or paper, rest each across two piles of books, and add coins one at a time until it sags. Keep the length and width the same so the test is fair, and record the results in a table.

Builds on

Key Stage 2: Strengthening and reinforcing structures
Key Stage 2: Gears and pulleys
Key Stage 2: Cams, levers and linkages
Key Stage 2: Series circuits in products
Key Stage 2: Programming and controlling products