thermoplasticity

Thermoplasticity is the temperature- and stress-dependent plastic deformation behavior of materials that can undergo permanent shape change when heated. In common usage, it describes thermoplastics, polymers that soften with heat and harden when cooled, allowing reshaping without chemical changes. The response is typically reversible through heating and cooling and can be repeated.

In polymers, the key transitions are the glass transition temperature Tg for amorphous plastics and the melting temperature Tm for crystalline or semicrystalline polymers. Above Tg, chain mobility increases and the material behaves as a viscous or viscoelastic liquid under sustained stress. Above Tm, crystalline regions melt and flow becomes easier. Processing methods include injection molding, extrusion, and thermoforming.

In metals and other materials, thermoplasticity describes temperature-activated plastic flow. Elevated temperatures reduce yield strength and enable greater dislocation motion, diffusion, and creep, allowing pronounced shaping or forming processes such as hot stamping or hot extrusion. The concept emphasizes that plastic deformation becomes easier as temperature rises, within material limits.

Thermoplastics contrast with thermosets, which cure into cross-linked networks that do not melt upon reheating. This distinction informs design, recycling, and processing choices. Challenges in thermoplasticity include thermal degradation, oxidative aging, and maintaining mechanical properties across service temperatures. Engineers use viscoelastic models and time-temperature superposition to predict behavior under real loading and thermal histories.