What Is a Thermoplastic?

A thermoplastic is a plastic that softens and melts when heated and hardens again when cooled, a cycle that is reversible and repeatable. Its long polymer chains are not chemically cross-linked, which is why thermoplastics such as ABS, polypropylene, polycarbonate and nylon can be injection molded, extruded, 3D printed and reground.

In short: heat it and it flows, cool it and it sets, and it can be melted again.

What Makes a Plastic a Thermoplastic?

It comes down to molecular structure. A thermoplastic is made of linear or branched polymer chains that are tangled together but held only by weak forces between the chains. Heating pushes the chains apart until they can slide past each other and the material flows. Unlike a metal, a thermoplastic does not melt at one exact temperature but over a range, typically about 15–20 °C (27–36 °F).

A thermoset is different. Its chains are tied together by strong covalent cross-links. Heat cannot pull them apart, so a cured thermoset never melts; heated far enough, it breaks down chemically instead.

Thermoplastics come in two broad structures:

• Amorphous, with randomly arranged chains, such as ABS and polycarbonate.
• Semi-crystalline, with ordered regions surrounded by amorphous material, such as nylon, POM (acetal) and polypropylene. For otherwise comparable polymers, semi-crystalline grades are typically stronger, stiffer and denser.

Thermoplastic elastomers (TPE, TPU) sit in between: they are processed like thermoplastics but behave like rubber in use.

Thermoplastic vs Thermoset

	Thermoplastic	Thermoset
Molecular structure	Linear or branched chains, no cross-links	Three-dimensional network of covalent cross-links
When heated	Softens and melts; reversible	Does not soften or melt; degrades if overheated
Can it be remelted?	Yes, many times, so scrap can be reground	No
Typical processes	Injection molding, extrusion, blow molding, thermoforming, FDM printing	Compression molding, casting, liquid injection and cure
Mechanical behavior	Often more ductile and tough	Generally harder and stronger, with better dimensional stability
Solvents	Can swell and, in a suitable solvent, dissolve	Can swell but cannot be fully dissolved
Examples	PE, PP, PVC, polystyrene, PTFE, ABS, PC, nylon, POM	Vulcanized rubber, epoxy, silicone, BMC/SMC compounds

How to Choose a Thermoplastic for a Part

• Check the temperature first. Near its glass-transition temperature a thermoplastic can lose one to two orders of magnitude of stiffness, which rules many grades out of hot, load-bearing jobs. Compare the service temperature with the grade's datasheet values.
• Check the chemicals. Solvents, fuels and even water can diffuse into some thermoplastics, swelling them and cutting stiffness and strength.
• Expect shrinkage to differ. Semi-crystalline grades shrink more in the mold than amorphous ones, which affects draft, tolerances and warpage.
• Prototype in the same family. An FDM print or machined block in the same polymer gives a truer early read on fit and function than a different material.

NICE Rapid's plastic injection molding service molds ABS, PP, PE, PC, nylon, POM, TPU, TPE and other engineering thermoplastics. Thermoset rubbers and BMC/SMC compounds are handled by compression molding instead.

Frequently Asked Questions

What is a thermoplastic?

A thermoplastic is a plastic that melts when heated and solidifies when cooled, and can go through that cycle repeatedly. Because its polymer chains are not cross-linked, it can be injection molded, extruded or 3D printed, and scrap can be reground and reused.

What is the difference between thermoplastic and thermoset?

A thermoplastic softens and melts when heated and can be remelted. A thermoset is cured into a cross-linked network, so once set it will not melt again; it degrades if overheated. Thermosets are generally harder and more dimensionally stable; thermoplastics are often tougher and easier to process.

What are common examples of thermoplastics?

Common thermoplastics include polyethylene (PE), polypropylene (PP), PVC, polystyrene, PTFE, ABS, polycarbonate (PC), nylon (PA) and acetal (POM). TPE and TPU are thermoplastic elastomers: processed like thermoplastics but flexible like rubber.

Related Services at NICE Rapid

• Plastic Injection Molding — thermoplastic parts from aluminum tooling for early batches to P20/H13 steel molds for production.
• LSR Molding — liquid silicone, a thermoset, including overmolding directly onto thermoplastic substrates.
• Request a Quote — tell us the service temperature and environment and our engineers will suggest candidate materials.

Sources

1. Penn State University, MATSE 81: Materials in Today's World, Lesson 8 "Thermoplastic and Thermosetting Polymers" — thermoplastics soften when heated and harden when cooled, reversibly and repeatably; most linear and branched polymers are thermoplastics; thermosets do not soften because of covalent cross-links and are generally harder, stronger and more dimensionally stable; PE, PP, PVC, polystyrene and PTFE as thermoplastic examples; vulcanized rubber as a thermoset — courses.ems.psu.edu
2. M.E. Tuttle, A Brief Introduction to Polymeric Materials, University of Washington (MEngr 553) — melting over about 15–20 °C; thermoplastic = meltable linear/branched polymer, thermoset = cross-linked polymer that cannot melt and degrades; thermoplastics can be melted and remelted many times and are often more ductile and tough; solvent plasticization and dissolution; semi-crystalline polymers stronger, stiffer and denser than comparable amorphous ones; stiffness loss of 1–2 orders of magnitude near Tg — courses.washington.edu
3. Michigan State University, ME 477 (P. Kwon), Plastics lecture notes — thermoplastic elastomers are processed like thermoplastics but used like elastomers; silicones listed among thermosetting polymers — egr.msu.edu