Choose a thermoplastic when the part may need to be remelted, reshaped or recycled, and a thermoset when the part must hold its shape in heat or carry electrical load. Thermoplastics such as celluloid soften when heated and harden when cooled, while thermosets such as Bakelite and hardened casein set permanently when cured, so a failed thermoset batch cannot be remelted and a thermoplastic one can.
Part of Plastics and Polymers
Thermoplastics and thermosets are the two families of plastic, distinguished by whether heating makes them soften reversibly or cure irreversibly.
Decide By What Happens To The Part Afterwards
This page is a selection guide. The materials-science articles cover each family in depth: Thermoplastics and Thermosets.
| Question | Thermoplastic | Thermoset |
|---|---|---|
| Can a mistake be melted and retried? | Yes | No |
| Will it keep its shape in heat? | Softens | Holds, then chars |
| Easier to make? | Easier: melt and mold | Harder: mix, then cure |
| Can scrap be reused? | Yes, by remelting | Only ground as filler |
| Typical hardness | Moderate | Hard, rigid |
| Early-plastic example | Celluloid | Bakelite, hardened casein |
Pair this with what Plastics from Scratch says: thermoplastics are useful for molding and forming, and thermosets are harder, more heat-resistant and better for electrical and structural applications.
Four Questions Settle It
- Will the part see heat in use? If so, a thermoplastic that softens may sag or fail. Bakelite, whose heat resistance the carrier gives as about 300 °C, is the answer. Treat that figure as the point where it chars, not a working temperature.
- Might you want to remake it? If so, a thermoplastic.
- Is it electrical? Bakelite. Celluloid is excluded by its flammability even though it is a thermoplastic.
- Do you have the means to cure? A thermoset needs a heated press and accurate heat control; a thermoplastic needs only to be heated and shaped.
How To Tell Which One You Have
Do not guess by heating. Heating an unidentified plastic to see whether it softens is the same hazard as burn-and-sniff: see Sorting and Identification. A better approach is to look at what the object is. Old dark switch plates, handles and brown-black moulded housings are usually phenolic thermosets. Clear or translucent bottles, jugs and packaging are thermoplastics. A part that is brittle, dark, hard and rings when tapped is probably a thermoset. Phenolic Resins lists identifying marks.
Hybrids And Edge Cases
- Casein is a thermoset only after hardening in formaldehyde. Unhardened casein is soft and takes up water.
- Vulcanised rubber is a thermoset: its sulfur bridges are permanent.
- Shellac is a natural thermoplastic that softens at about 65 °C, per the corpus.
Worked Example: Choosing For A Switch Body
A switch body sits next to conductors that warm up. It must not sag, burn or leak. Celluloid fails on flammability. A thermoplastic with a working range of 120-180 °C risks softening near a hot contact. Bakelite fits: insulating, heat-resistant, rigid. The cost is that a cracked body cannot be remelted, so you mold a few spares in the first run.
Common Mistakes
- Using a thermoplastic where heat is routine.
- Planning to rework a thermoset.
- Choosing by ease of making alone — a forgiving material that fails in service is not an economy.
Heat is the test and the hazard
Do not use heat to find out which family a plastic belongs to. Heated unidentified plastic can release hydrogen chloride, hydrogen cyanide or an acutely toxic fume, and phenolic scrap releases phenol and formaldehyde. See Fume Safety.
What’s Next
- Learn next: Casein: Applications and Limits — a borderline case
- Combine with: Bakelite — the thermoset to start with