Heat can be described physically as oscillation of the molecules.
An excessive oscillation can impact the molecules of plastic drastically.
The effect of it is called thermal impact.
In the worst case, the molecules can break: the molecules degrade.
This is a damage of the plastic, which increases the higher the prevailing temperature.
Furthermore, the duration of the temperature effect – in injection molding called residence time – plays an important role.
An example from the daily life is the sunburn.
Here again, the two factors temperature and residence time are responsible for the thermal impact.