What is the Heat Distortion Temperature (HDT) and how does it relate to heat resistance?
HDT is the property of a polymer system that can be accurately determined. It is the initial softening point of the Polymer and relates closely to the glass transition temperature of the polymer. It is determined in accordance with ASTM D648 and involves supporting a standard cast bar of the material at each end and loading the centre at 264 psi fibre stress. The sample is then warmed in an oil bath at a rate of 2°C per minute until a deflection of 0.025mm (0.010ins.) occurs. The recorded temperature is the HDT or Deflection Temperature. Mechanical strength will progressively deteriorate above the HDT. Heat resistance is a less easily defined point at which the product will no longer perform in a satisfactory manner and will depend on several factors other than just heat. Ultimate heat resistance may be defined as the temperature above which irreversible degradation occurs to the material. Up to this temperature and upon cooling the material will revert back to an equal or slightly str
