Plastics
Plastics are made from oil or gas by joining small molecules into long chains. They are light, waterproof, cheap, and do not rust or rot. They can be coloured and moulded into almost any shape.
Two families
Thermoplastics (like PET bottles, polythene, acrylic) go soft when heated and hard when cooled. They can be re-melted and recycled. Thermosetting plastics (like bakelite and melamine) are shaped once with heat. After that they stay hard and do not melt again; they char instead. They are used for plug sockets and handles.
How plastic is processed
Injection moulding: melted plastic is pushed into a metal mould (toys, buckets). Blow moulding: air blows plastic into a bottle shape. Extrusion: plastic is pushed through a shaped hole to make pipes. Vacuum forming: a warm sheet is sucked over a shape. Plastics do not rot, so they stay in nature for a long time: reuse and recycle them.
Timber
Timber is wood from trees that has been cut into planks. It is made of long fibres that run along the tree. These fibres make a pattern called the grain. Timber is strong along the grain and weak across it, and it splits easily along the grain.
Hardwood and softwood
Hardwood comes from broad-leaved trees (teak, sheesham, oak). It is dense, strong and slow-growing. Softwood comes from cone-bearing trees (pine, deodar). It is lighter, cheaper and grows fast.
Preparing and using timber
Fresh timber holds water. It is dried, or seasoned, in air or in a kiln, so it does not shrink, crack or warp later. Then it is sawn, planed, joined and finished with polish or paint. Manufactured boards such as plywood and MDF use thin layers or fibres glued together and are more stable than solid wood. Timber is renewable if new trees are planted, and it can burn, rot or be eaten by insects unless treated.
Plastic or timber: choosing
Choose plastic for waterproof, light, moulded parts. Choose timber for stiff, warm-looking, repairable furniture and frames. Ask: what load, what heat, what water, how long, and what happens to it at the end of its life?
Try it: grain test
In the 3D, pick Timber with grain along, then across, and raise the load. Which one splits first? At home, try to snap a wooden ice-cream stick: it breaks easily across the grain but not along its length. Also note how a warm plastic spoon bends but a cold one does not.
Key formulas and definitions
- Thermoplastic: soft when hot, hard when cold, can be re-melted
- Thermoset: shaped once, stays hard, will not re-melt
- Timber: strong along the grain, weak across it
- Hardwood = dense, slow-growing; softwood = light, fast-growing
- Seasoning timber = drying to reduce water so it does not warp
Worked examples
1. Why are plug sockets made of thermosetting plastic?
They must not soften when they get warm. A thermoset keeps its shape and does not melt again after it is set.
2. A shelf plank is cut so the grain runs across its length direction. What goes wrong?
The grain across the span makes the plank weak. It can split at a small load. Cut it so the grain runs along its length.
3. Why is timber seasoned before furniture is made?
Wet timber shrinks and warps as it dries. Seasoning removes water first, so the finished furniture keeps its size and shape.
Common mistakes
- Thinking all plastic can be re-melted. Thermosets cannot.
- Cutting timber with the grain across the load. It splits easily.
- Using fresh, unseasoned wood for furniture. It warps and cracks.
- Saying hardwood is always harder than softwood. The names come from the tree type, and a few softwoods are harder than some hardwoods.