Thermoplastic resins melt when its temperature becomes higher than its melting point and becomes liquid, but as it is gradually cooled from the liquid state its viscosity gradually increases, and it goes into a rubbery state and it finally solidifies. The temperature at the border from the rubbery state to the solid state (called the glass state) is called its "glass transition temperature". The glass transition temperature is generally indicated as Tg.

Tg is a physical quantity that is determined by the type of plastic. By knowing the value of Tg, it is possible to estimate the time required for the molded item to cool inside the cavity and the molded item temperature at which it can be ejected from the mold.

The values of Tg are given below for the major plastics.