Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of glass transition temperature (tg). No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
For an amorphous plastic, Tg is the real service ceiling — stiffness falls off a cliff above it. For a semi-crystalline plastic it matters much less, because the crystalline regions keep the material stiff well past Tg.
This is why nylon 6/6 has a Tg of only 50 °C yet works at 105 °C, and why the melting point is not a service limit either. The number to design against is the deflection temperature under load.
Common misuse
Using Tg as the service limit for a semi-crystalline plastic. It is the limit for amorphous ones; semi-crystalline plastics keep working well above it.