Glossary · Value

3D printing

Glass transition temperature

The temperature at which a plastic changes from hard and glassy to soft and rubbery. For PLA it is only around 55 to 60 °C.

Explained in more detail

At the glass transition, the disordered, amorphous regions of a plastic become mobile, and stiffness and strength drop markedly. Unlike a melting point, the glass transition is not a sharp point, and the measured value depends on the method, such as DSC or DMA. The temperature is abbreviated Tg. For 3D printing it matters for two reasons: above it, parts deform under load, and when drying filament the temperature must stay below it, otherwise the windings stick together on the spool. Semi-crystalline plastics such as PA or PP additionally have a melting range for their crystalline portions.

Where does the term come from?

The name comes from glass: window glass is also amorphous and softens gradually when heated instead of melting at a fixed point. In English the value is called glass transition temperature.

In practice

• Do not use PLA parts in a car, on a sunny windowsill or near a heater, because the glass transition is quickly reached there.
• When drying, stay below the glass transition temperature, and for PLA 45 to 55 °C is usual.
• In enclosed printers with PLA, open the door or lid, otherwise the filament softens before it reaches the nozzle.
• For warm environments, choose materials with a higher glass transition, such as PETG, ASA or PC.

Not to be confused

Do not confuse it with heat deflection temperature (HDT), which is measured under load. And not with the printing temperature: a plastic only becomes printable well above the glass transition.

Frequently asked questions

At what point does PLA soften?

From around 55 to 60 °C, in the region of the glass transition temperature. Above that, parts deform under load or under their own weight.

Why does a filament not list a glass transition temperature?

Many manufacturers give the heat deflection temperature (HDT) or the Vicat temperature instead. Only values measured with the same method are comparable.