Glass transition is a reversible transition in amorphous materials from a hard and brittle “glassy” state into a viscous or rubbery state as temperature is increased. The temperature that this occurs at is referred to as the glass-transition temperature, Tg. The Tg is always lower than the melting temperature, Tm, at which point the material changes state from a solid to liquid form.
This applies to polymers, including those used in 3D printing, and is useful to know and understand the Tg-Tm temperature ranges for managing the printing process as the solid filament is melted in the extruder before leaving the nozzle and cooling into a workable viscous state. I’ve typically seen the print temperatures set to just above the Tm, and print bed temperatures just below Tg, and I will need to see how well that works out in practice. It is additionally important to be aware of the Tg when considering functional uses and storage of printed objects, for example PLA has a relatively low Tg that makes it unsuitable for hot cars or use in cooking since it can start to lose its shape and structural integrity.
Here are some example Tg-Tm ranges, which may vary depending on the specifics of the material formulations: PLA with Tg 60–65 °C and Tm 180–220 °C, ABS with Tg ~105 °C and Tm taken effectively as ~230 °C (the Vicat softening temperature, the softening point for materials without a definite melting point), and TPU (thermoplastic polyurethanes, elastic at room temperature) with Tg -53 – -28 °C and Tm (Vicat) 88–147 °C.
References:
Modeling the Glass Transition of ABS in a 3D Printer
Thermoplastic Polyurethane (TPU) Typical Properties Generic TPU Alloy