1.1.24 Thermoforming & Thermosetting Plastics
D&T 2026 — 1.1(c) Product Design Pathway
Aesthetic and functional properties of key polymers
Polymer — a material made of long chain molecules. Most are synthetic plastics made from crude oil; a growing number of biopolymers are made from plants.
| Polymer | Type | Aesthetic properties | Functional properties | Typical uses |
|---|---|---|---|---|
| Acrylic (PMMA) | Thermoforming | Glass-like clarity or bright colours; high-gloss surface; can be frosted or mirrored | Hard and rigid; weather resistant; scratches fairly easily; brittle and can crack if hit or drilled carelessly; laser cuts cleanly with polished edges | Signs, display stands, light fittings, shop fittings, car light covers, laser-cut products |
| Styrofoam (extruded polystyrene) | Thermoforming | Light blue, smooth and fine-celled | Very light; easy to cut, carve and sand to shape; good insulator; dissolved by solvents and some paints | Block models and prototypes, building insulation |
| Modelling foams (e.g. polyurethane modelling board) | Usually thermosetting | Fine, even texture that sands very smooth | Denser and harder than Styrofoam, so holds sharp detail; can be machined on CNC routers; takes paint well | High-quality presentation models, moulds and patterns |
| ABS | Thermoforming | Glossy or textured surface from the mould; available in any colour | Tough and impact resistant; rigid and hard-wearing; injection moulds well; 3D prints well | Electronic casings, LEGO bricks, toys, kitchen appliances, helmets |
| Nylon (polyamide) | Thermoforming | Creamy white or coloured; can be translucent | Tough and hard-wearing; low friction (self-lubricating); good chemical resistance; can absorb moisture | Gears, bearings, cable ties, zips, curtain rails, clothing and rope fibres |
| Epoxy resin | Thermosetting | Clear or coloured; glossy when set | Two-part (resin + hardener); very strong adhesive; hard, rigid and chemically resistant when cured; good electrical insulator | Strong adhesives, the matrix in carbon-fibre composites, castings, encapsulating electronics, PCB boards |
Styrofoam — the trade name for extruded polystyrene foam, the blue modelling foam used in schools. It is not the same as expanded polystyrene (EPS), the white beaded foam used for packaging.
When cutting or shaping Styrofoam with a hot wire cutter, work in a well-ventilated area, because the melting polymer gives off fumes.
Thermosetting and thermoforming plastics
Thermoforming plastic — sometimes called a thermoplastic: a polymer that softens when heated and can be reshaped many times, because its long chains are not bonded to each other.
Thermosetting plastic — a polymer that sets permanently when first heated or cured. Strong chemical cross-links form between the chains, so it cannot be softened or reshaped again.
| Thermoforming plastics | Thermosetting plastics | |
|---|---|---|
| Advantages | Can be reheated and reshaped (vacuum forming, line bending, injection moulding); can usually be recycled; wide range of colours and finishes; mistakes can be reheated and corrected | Stay rigid and keep their shape at high temperatures; hard and scratch resistant; excellent electrical and heat insulators; chemically resistant |
| Disadvantages | Soften and lose their shape when hot; often less scratch resistant; some become brittle in UV light | Cannot be reshaped once set; very difficult to recycle; often brittle; mistakes cannot be corrected |
| Examples | Acrylic, ABS, nylon, HDPE, polypropylene, PVC | Epoxy resin, polyester resin, melamine formaldehyde, urea formaldehyde |
Physical properties: thermal conductivity, electrical conductivity and insulation
Most polymers are poor conductors of heat and electricity, which makes them excellent insulators.
Thermal insulator — a material with low thermal conductivity, which slows the flow of heat through it.
- Thermal insulation — polymer handles on saucepans and kettles stay cool enough to hold. Foamed polymers such as Styrofoam and EPS are especially good insulators because they trap air in tiny cells, so they are used for building insulation and cool boxes.
- Electrical insulation — polymers are used for cable sheathing, plugs, switches and the casings of electrical products, protecting users from electric shock. Thermosets such as urea formaldehyde are used for plugs and sockets because they also resist heat and will not soften if a connection gets warm.
- Exceptions — special conductive polymers have been developed that do conduct electricity. They are used in flexible electronics and touch-sensitive surfaces (see 1.1.25 Modern, Smart & Emerging Materials).
Mechanical properties of polymers
| Property | Meaning for polymers | Example |
|---|---|---|
| Tensile strength | Resistance to being pulled apart | Nylon rope and cable ties |
| Elasticity | The ability to stretch or bend and return to the original shape when the force is removed | Polypropylene living hinges on shampoo bottle lids |
| Toughness | The ability to absorb impact without cracking | ABS phone cases and helmets |
| Plasticity | The ability to be permanently reshaped without cracking — thermoforming plastics show high plasticity when heated | Acrylic softened on a strip heater and bent to shape |
| Malleability | The ability to be pressed or formed into shape without breaking | Heated polystyrene sheet pulled over a mould in vacuum forming |
| Hardness | Resistance to scratching and wear | Melamine worktops; acrylic is harder than polythene but still scratches |
Biopolymers compared with synthetic oil-based polymers
Biopolymer — a polymer made from renewable biological sources such as corn starch, sugar cane or cellulose, rather than from crude oil.
The best-known example is PLA (polylactic acid), made from plant starch and widely used as 3D printing filament, disposable cups and food packaging.
| Biopolymers (e.g. PLA) | Synthetic oil-based polymers (e.g. ABS, polypropylene) | |
|---|---|---|
| Source | Renewable plant crops | Finite crude oil |
| Carbon footprint | Usually lower — plants absorb CO₂ as they grow | Higher — extracting and refining oil releases CO₂ |
| End of life | Many are compostable, but most need the high temperatures of an industrial composting facility, not a home compost heap or landfill | Can often be recycled, but can last hundreds of years in landfill or the sea |
| Properties | Often more brittle and less heat resistant (PLA softens at about 60°C) | Wide range of strong, tough, heat-resistant grades |
| Cost | Usually more expensive | Usually cheaper |
| Other issues | Growing the crops uses farmland, water and fertiliser that could produce food | Microplastic pollution; dependence on fossil fuels |
“Biodegradable” does not mean a product will break down anywhere. Many biopolymers only compost in industrial facilities, and they can contaminate normal plastic recycling if put in the wrong bin.
How surface treatments and finishes prolong material life
Many polymers are self-finishing — the shape, colour and texture come straight from the mould, so no further finish is needed. Polymers do not rust, but they still degrade over time:
- Ultraviolet (UV) light makes many polymers fade, discolour and become brittle. UV-resistant coatings and additives (UV stabilisers) slow this down, which is why garden furniture and car parts last longer outdoors.
- Scratches and wear spoil the appearance of clear polymers. Hard coatings on acrylic and polycarbonate (for example on spectacle lenses and visors) improve scratch resistance.
- Cut edges are polished (wet-and-dry paper, polishing compound or flame polishing on acrylic) so they look better and are less likely to crack from small nicks.
- Paint and printed graphics can add colour or branding, but most polymers need a suitable primer or surface preparation so the coating sticks.
See 1.1.27 Surface Finishes & Treatments for finishes on all material groups.