Heat Deflection Temperature (HDT) and Vicat Softening Temperature Testing

HDT stands for Heat Deflection Temperature. The heat deflection temperature of a reinforced or unreinforced polymer material is a measure of polymer’s resistance to distortion under an applied load at elevated temperatures.

Applications of HDT tests include:

1) Identify suitable material for injection molding application.
2) Identify suitable material for elevated temperature application.
3) Identification and grading of materials as per their properties.

The test specification for the HDT is ASTM D 648 and ISO 75.

Factors Influencing Thermal Performance of Polymer Materials

HDT tests typically test for the short term performance of the materials under loads at elevated temperatures. The following factors play a significant part in the performance prediction of the materials under the test conditions.

1) The total time material is exposed to elevated temperatures.
2) The rate of temperature increase.
3) The specimen dimensions and part geometry.

Vicat softening temperature tests are used to identify the temperature at which a needle of specified dimensions penetrates into a plastic material specimen for a specified distance under applied loading conditions. The test specification for the Vicat softening temperature testing is ASTM D1525 and ISO 306.

Compared with the Heat Deflection Temperature (HDT), Vicat softening temperature test measures the temperature at which the specimen loses its stiffness and softens. HDT test measures the temperature at which the specimen loses its load bearing capability. The Vicat softening point is closer to the actual melting or softening point of the polymer. Vicat softening temperature is typically always higher than the HDT for a polymeric material as the penetration load is always higher than a bending load on a material specimen.