GCSE Design & Technology: Timbers, Metals and Polymers Revision Guide

Timbers

Timber is divided into hardwoods and softwoods. Hardwoods, such as oak, beech and mahogany, come from slow-growing deciduous (broad-leaved) trees and are generally dense, strong and durable, making them suitable for furniture and flooring, although they tend to be more expensive. Softwoods, such as pine, spruce and larch, come from fast-growing coniferous trees. They are usually cheaper and easier to work but less durable, so they are widely used for construction timber, fencing and inexpensive furniture. Manufactured boards, including plywood, MDF (medium density fibreboard) and chipboard, are produced from natural wood waste bonded with resin. Plywood is made from thin layers of veneer glued with the grain running at right angles to add strength and stability in all directions; MDF is made from fine wood fibres bonded under heat and pressure to give a smooth, easily machined surface; chipboard is made from wood chips and resin and is cheap but weak, often used as a core material with a veneer or laminate finish.

Metals

Metals are divided into ferrous and non-ferrous types. Ferrous metals, such as mild steel, high carbon steel and cast iron, contain iron and are generally strong but prone to rusting because iron reacts with oxygen and moisture. Non-ferrous metals, such as aluminium, copper and tin, contain no iron, so they resist corrosion, and they are often lighter. Alloys are mixtures of two or more metals (or a metal and another element) designed to combine useful properties — for example, brass (copper and zinc) is more durable and easier to cast than pure copper, and stainless steel (steel with added chromium) resists corrosion far better than mild steel alone.

Polymers

Polymers are divided into thermoplastics and thermosetting polymers. Thermoplastics, such as acrylic (PMMA), polypropylene (PP) and high-density polyethylene (HDPE), soften when heated and can be reshaped and remelted repeatedly, which makes them relatively easy to recycle. Thermosetting polymers, such as epoxy resin, melamine formaldehyde and urea formaldehyde, undergo a permanent chemical change when first moulded, forming strong cross-links between molecules. Once set they cannot be melted and reshaped again, which makes them heat-resistant and hard-wearing but harder to recycle.

Working properties

When selecting a material, designers consider its working properties. Hardness resists scratching and wear; toughness resists sudden impact without shattering; malleability allows a material to be hammered or pressed into shape without cracking; ductility allows a material to be drawn out into a wire; and elasticity allows a material to return to its original shape after being deformed. Density, strength and corrosion resistance are also key factors, alongside cost and availability, when choosing the most appropriate material for a product.

Common mistakes

• Assuming all hardwoods are physically harder than all softwoods — the terms actually refer to the type of tree, not the hardness of the wood (balsa, for example, is a very soft hardwood).
• Confusing malleability (being hammered into shape) with ductility (being drawn into a wire) — these are different properties and are often mixed up in exam answers.
• Saying thermosetting polymers can be recycled by remelting them in the same way as thermoplastics, when in fact their permanent chemical bonds mean they cannot be remelted once set.