Common Materials: Wood, Metal and Plastic
FoundationMaterials are natural or man-made by origin. Wood, metal and plastic each have their own properties and uses; choosing them by strength, weight, cost and looks is the first step of Design & Technology.
- Materials are grouped by origin into natural and man-made
- Wood is light and easy to work; metal is strong and conducts; plastic is light, waterproof and mouldable
- Choose materials by strength, weight, cost, looks, durability and the environment
文體小知識：科技知識說明文用客觀、準確的文字講解科技概念、材料與原理，讓讀者清楚明白事物是什麼和為什麼。
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1Look around: classroom desks, the aluminium windows at home, the buckles on your schoolbag — all are made of materials. A material is the basic substance used to make a product, and resources are where these materials come from. Knowing the types and properties of materials, and why a product uses a particular one, is the first step of Design & Technology and knowledge you need before making anything.
2By origin, materials fall into two groups: natural and man-made. Wood, stone, cotton and natural rubber come directly from nature with only simple processing; plastics, glass and most alloys are made by people from raw substances. Most metals, such as iron, aluminium and copper, are extracted from ores. Classifying materials by origin and properties is a basic skill when choosing them.
3Wood, metal and plastic are the three most common groups in daily life. Wood is light, has a natural grain and is easy to cut, but it fears moisture and insects. Metal is hard and strong and conducts heat and electricity, so it is widely used for structures. Plastic is light, waterproof and an insulator; when heated it can be moulded into almost any shape, which makes it the star of packaging and household goods.
4When choosing a material, consider strength, weight, cost, durability, appearance and the environment. For example, the window frames of tall buildings use aluminium alloy because it is light, rust-free and durable, while toys are mostly plastic because it is cheap and easy to mould. Knowing about composite materials and recycling also helps us design products that are practical and environmentally friendly.
1Summary 重點摘要
Materials are natural or man-made by origin. Wood, metal and plastic each have their own properties and uses; choosing them by strength, weight, cost and looks is the first step of Design & Technology.
2Vocabulary cards Vocabulary
The basic substance a product is made from.
A material from nature with simple processing only, such as wood and cotton.
A material made by people from raw substances, such as plastic and glass.
A natural material from trees; light and easy to work with.
A hard, strong material that conducts heat and electricity, e.g. iron, aluminium, copper.
A light, waterproof, insulating man-made material that can be moulded when heated.
A built-in characteristic of a material, such as strength, hardness, weight and conductivity.
A material combining two or more materials to take the best of each, e.g. fibreglass.
The ability of a material to resist wear and ageing over long use.
Turning waste materials into new products to save resources.
3Step by step Steps
- What materials and resources areMaterials are the basic substances that make up every product, while resources are where those materials come from, whether natural or man-made. Before designing anything, decide which material to use, where it comes from and whether it can be recycled. For example, a plastic lunch box is heated and pressed from plastic pellets made from oil, while the aluminium of a window frame is extracted from bauxite ore. Behind both lie precious resources of the Earth, and using them sparingly is a designer's duty.
- Natural versus man-madeBy origin, materials divide into natural and man-made. Natural materials come directly from plants, animals and minerals — wood, cotton, wool, stone and natural rubber — needing only simple cutting or cleaning. Man-made materials are synthesised by chemical or physical processing, such as plastics, glass and synthetic fibres. Metals come from ores but require smelting and other processing before use. For example, wooden chopsticks are natural while plastic chopsticks are man-made: their sources and processing differ completely.
- Wood: properties and usesWood is light, has a natural grain and feels warm, and it is easy to saw, nail and carve — a historic material for buildings and furniture. Its weaknesses are fear of moisture, rot and insects, easy burning and strength lower than metal. Common timbers divide into solid wood and manufactured boards. For example, classroom desks, home floors and wooden doors use wood for its warm look and easy working, while cupboard doors often use plywood or chipboard, which cost less and resist warping.
- Metal: properties and usesMetal is hard, strong and heat-resistant, conducts electricity and heat, and can be cast, forged, welded and recycled — the star of structures and appliances. Common metals include steel, aluminium and copper: steel goes into reinforcing bars and bridges, aluminium is light and rust-free, and copper is first choice for wires. Metals are heavier, some rust, and they cost more. For example, MTR train bodies use aluminium alloy to cut weight, wires use copper for its conductivity, and kitchen pots use stainless steel for durability and hygiene.
- Plastic: properties and usesPlastic is a synthetic polymer whose great merits are being light, waterproof, insulating and corrosion-resistant; heated, it can be moulded into almost any shape at low cost, suiting mass production. Its weaknesses are poor heat resistance, moderate strength, slow decomposition after disposal and pollution if handled badly. Plastics come in many kinds with different properties. For example, carrier bags use polyethylene, food boxes use heat-tolerant polypropylene, and acrylic is clear like glass — always check whether a plastic is suitable for hot food.
- Choosing and combining materialsIn design, materials are chosen by their properties: strength, weight, cost, looks, durability and environmental impact — and different materials are often combined so each does what it does best. Knowing synthetic, composite and biological materials widens the choice. For example, an umbrella uses metal ribs to resist wind, waterproof fabric for the canopy and a plastic handle for insulation and grip. With the three materials each playing its part, the product is handy and long-lasting.
4Diagrams Diagrams
5Worked example Example
Example: your class teacher wants a bookshelf for the reading corner on a small budget. Wood, metal or plastic — which would you suggest?
- List the needs → must hold many heavy books, used daily, small budget, moved by students
- Metal (aluminium or steel) → strong and durable, but heavier, costlier and needs special equipment
- Plastic → light and cheap, but not strong enough; shelves would bend under heavy books
- Wood (solid wood or plywood) → strong enough, easy to saw and assemble, affordable, warm look
- Answer: Wood (plywood with a solid-wood frame) is suggested. Books are heavy, so plastic would bend; metal is strong but costs more and needs special equipment to work. Wood is strong enough, easy to saw and nail, affordable for students to make in the workshop, attractive in grain and durable once painted. Choosing materials is about balancing properties against needs — never assume one material is always best.
6Common mistakes Mistakes
7Quick check Quick check
Q1Which group is all natural materials? A. wood, cotton, stone B. plastic, glass, aluminium C. cotton, plastic, wood
Q2Why are the window frames of tall buildings usually made of aluminium alloy?
Q3Why is copper, not plastic, used inside electric wires?
Q4What problem may appear if a bookshelf for heavy textbooks is made of plastic board?
Q5Give one household use each for wood, metal and plastic, with a reason.
8Recap Recap
- Materials are grouped by origin into natural and man-made
- Wood is light and easy to work; metal is strong and conducts; plastic is light, waterproof and mouldable
- Choose materials by strength, weight, cost, looks, durability and the environment
- Composite materials combine the strengths of several materials
- Knowing material properties is the first step of design and making