AQA A-Level Design and Technology: Product Design

Choosing a material

A designer names a material for a product and then defends the choice by weighing several things against each other: the physical and mechanical properties (the working characteristics), what the product has to do, how it should look and feel, what the material and its processing cost, and what happens to it when the product is made and when it is thrown away. A material that is ideal on one of these is often poor on another. Carbon fibre reinforced plastic is stiff and light but expensive and hard to recycle, so it suits a racing bicycle and not a school ruler. Pine is cheap and easy to work but decays outdoors unless it is treated, so it suits a shed frame once it has been preservative treated and not a boat hull.

Combinations matter as much as single materials. A kettle uses a polymer body, a metal element, a rubber or elastomer grip and a thermostat, and each part is chosen for its own job. A good justification therefore names the property, says why the product needs it, and says what is given up.

Typical physical and working characteristics that come up are malleability, toughness, hardness, resistance to corrosion and degradation, thermal conductivity and electrical conductivity.

The material families

Metals

Metals are ductile and malleable to varying degrees, strong, good conductors of heat and electricity, and can be joined by heat processes and alloyed. They are split into three groups.

• Ferrous metals contain iron as the main element and are generally magnetic and prone to rusting, apart from stainless steel. Examples are low carbon steel, medium carbon steel, high speed steel and cast iron.
• Non-ferrous metals contain no iron, so they do not rust, and are generally non-magnetic. Examples are aluminium, copper, zinc, silver, gold, titanium and tin.
• Alloys are a mixture of a metal with one or more other elements, usually other metals, made to improve properties such as strength, hardness or corrosion resistance. Brass (copper and zinc) and bronze (copper and tin) are non-ferrous alloys; stainless steel and die steel are ferrous alloys.

Woods

• Hardwoods come from broad-leaved, mostly deciduous trees such as oak, ash and beech. They grow slowly, so they are generally denser, more durable and more expensive. The names describe the type of tree and not always the actual hardness: balsa is a hardwood and very soft.
• Softwoods come from conifers such as pine, spruce and larch. They grow faster, so they cost less and are widely used for construction, though many need preservative treatment outdoors.
• Manufactured boards are made from veneers, chips or fibres bonded with adhesive. They come in large, stable, flat sheets that do not warp as solid timber does. Plywood, chipboard and medium density fibreboard (MDF) are the standard examples.

Polymers

• Thermoplastics soften when heated and can be reshaped again and again, which makes them recyclable and suited to moulding processes. Acrylic, polypropylene and ABS are examples.
• Thermosetting polymers (thermosets) undergo a chemical change during their first heating and then set permanently. They cannot be softened again, they char at high temperature, and they resist heat well. Urea formaldehyde and epoxy resin are examples.
• Elastomers can be stretched a long way and return to their original shape. Natural rubber and silicone are examples.

Papers and boards

Papers and boards are made from cellulose fibres, mostly from wood pulp, felted into sheets. The usual dividing line is thickness or weight: a sheet over about 200 grams per square metre is normally called a board or card. They are cheap, easy to cut, fold and print, and lightweight, so they are the main materials for packaging and for modelling.

Composites

A composite is made by combining two or more different materials so that the result has properties neither has alone. A typical composite has a matrix that holds the shape and a reinforcement that carries load. Glass reinforced plastic (GRP), carbon fibre reinforced plastic (CFRP) and reinforced concrete are examples. Plywood is also a composite, built from layers of wood and adhesive.

Smart materials

A smart material responds to a change in its surroundings, such as temperature, light or pressure, in a predictable and reversible way, and returns to its first state when the stimulus is removed. A thermochromic pigment changes colour with heat, and a shape memory alloy returns to a set shape when warmed.

Modern materials

A modern material is one developed through recent invention or the improvement of an older material, giving properties that were not previously available. Kevlar, precious metal clay and polymorph are examples. The difference from a smart material is that a modern material does not need to react to anything: its useful property is simply there. The line between the two groups can blur, and the point for a designer is to name the property being used.

Putting examples in the right group

A frequent task is to classify a named material or to suggest one from a group. Some quick checks help.

• If a magnet sticks and the metal is not stainless steel, it is ferrous and will rust unless it is protected.
• If a polymer can be heated and reshaped, it is a thermoplastic; if it chars instead of softening, it is a thermoset.
• If a material is made of two distinct parts that can be seen or described separately (fibre and resin, steel and concrete), it is a composite.
• If a material changes in response to heat, light or pressure and then goes back, it is smart.
• Stainless steel appears in two lists in common usage. It is a ferrous alloy because it is iron-based and an alloy because chromium is added.

Material choice in a worked context

For a child's toy that is chewed, dropped and washed, a designer might compare a hardwood such as beech, polypropylene and an elastomer such as silicone. Beech is hard, close-grained and has no toxic additives, but it can splinter and needs a safe finish, and it cannot be cleaned in a dishwasher. Polypropylene is lightweight, tough, cheap to injection mould in volume and dishwasher safe, but it is made from oil and its recycling depends on local collection. Silicone is soft, non-toxic, heat resistant and gentle on gums, but it costs more and is harder to recycle. The justification weighs these in terms of the product's function, safety, cost, appearance and disposal, rather than simply naming the "best" material.