The Heat Distortion Temperature for a material can be determined experimentally.
ASTM D8 defines the Heat Distortion Temperature as the temperature at which a bar of standard dimension experiences a deflection of .25mm when loaded with a flexural load of 455 kPa. The heating is done at the rate of 2K per minute.
A material cannot be used for structural applications at a temperature above its Heat Distortion Temperature.
In Injection moulding, the part which is moulded can be removed from the mould once it is near or below the Heat Distortion Temperature.