Thermoforming & Thermosetting Polymers
GCSE — 2.1.2 In-depth Knowledge: Product Design
Differences between thermoforming and thermosetting
Thermoforming (thermoplastic) — materials that can be heated and shaped repeatedly, and are able to be recycled.
Thermosetting polymer (thermoset) — materials that are formed once and cannot be reheated, remoulded or recycled.
The main difference: when a thermosetting polymer is first heated and set, a chemical change forms permanent cross-links between its molecule chains, so it cannot be softened or remoulded again. Thermoforming polymers have no cross-links, so their chains can slide past each other when heated, letting them be reshaped many times.
Thermoforming polymers
| Polymer | Properties | Uses |
|---|---|---|
| Acrylic | Hard, good plasticity when heated, weather resistant, easy to scratch, can be brittle | Car headlights, protective visors, baths, signs |
| Polypropylene (PP) | Lightweight, strong, tough, good heat and chemical resistance | Game cases, patio chairs, children’s toys, food wrapping film |
| PVC (Polyvinyl chloride) | Low-cost, flexible or rigid, glossy or matt, good strength and weather resistance | Window/door frames, guttering, building cladding |
| HDPE (High density polythene) | Stiff, lightweight, excellent chemical resistance, good plasticity when heated | Buckets, bottles, pipes, washing up bowls |
| Polystyrene (PS) | Inexpensive, available in opaque colours and transparent | Vacuum forming in schools, packaging |
| Nylon | Tough, self-lubricating, resistant to wear | Gears, bearings, clothing |
| ABS | Strong, tough, impact resistant | Casings for electronics, helmets, LEGO |
| Polycarbonate (PC) | Transparent, extremely tough and impact resistant (far tougher than acrylic), good heat resistance; injection mouldable | Safety glasses, riot shields, roof sheeting, clear casings |
| LDPE (Low density polythene) | Flexible, tough, good chemical resistance, low cost | Carrier bags, squeezy bottles, cling film, toys |
| Expanded polystyrene (EPS) | Lightweight white beaded foam, excellent insulator, absorbs shock | Protective packaging, disposable cups, insulation |
Thermosetting polymers
| Polymer | Properties | Uses |
|---|---|---|
| Melamine formaldehyde | Excellent resistance to heat, moisture, scratching and staining | Kitchen worktops, tableware |
| Epoxy resin | Two-part (resin + hardener), very strong adhesive, good chemical and heat resistance | Adhesive, casting, composites |
| Urea formaldehyde | Hard, stiff, excellent electrical insulation | Switches, plugs, electrical fittings |
| Polyester resin | Hard, stiff, good chemical and weather resistance when set; two-part (resin + catalyst) | Glass reinforced polymer (GRP) boat hulls and car body panels, casting |
Sources of polymers
Polymers can be made from natural and synthetic resources:
- Synthetic polymers — made from crude oil by scientists and engineers
- Natural polymers — made using materials like silk, wool, cellulose and proteins
Availability of polymers
Polymers are available in a variety of forms:
- Pellets, granules, powder — melted and formed into shapes
- Foams, films, filaments — blown or rolled
- Sheets, rods, bars — standard stock forms
Finishing of polymers
- Some polymers formed in a mould are self-finishing — the surface of the mould dictates the finish, requiring no further treatment
- If cut from stock (e.g. acrylic from a sheet), cut edges will be rough and need finishing
- Other finishes include: paint spraying, rubberising sprays, heat transfer printing
- Decorative additions: vinyl cut graphics, flocking, laser etching, decals
Properties of polymers
The selection of a polymer for a purpose relates to:
- Cost
- Resistance to corrosion
- Strength and strength/weight ratio
- Conductivity
- Elasticity, stiffness or malleability