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Determination of Heat Deflection Temperature (HDT) and Vicat Softening Temperature (VST)

ISO 306, ASTM D1525, ISO 75, ASTM D648

Thermoplastics do not exhibit an exact melting point that indicates the precise transition from the solid to the liquid state. Instead there is a gradual softening as the temperature increases.

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Vicat softening temperature VST ISO 306 and ASTM D1525

Therefore, the Vicat softening temperature is used in measuring technology as a replacement value for the melting point. It indicates the temperature at which a circular indenter with a cross-section of 1 mm² under a standard load of 10 N or 50 N will penetrate the specimen precisely 1 mm deep. The Vicat softening temperature is defined in ISO 306 and ASTM D1525.

Heat Deflection Temperature (HDT) ISO 75 and ASTM D648

Another measurand is the heat deflection temperature (HDT) at which a prismatic specimen is subjected to bending and warmed to a defined temperature increase. Once the heat deflection temperature is reached, the specimen has achieved a deflection defined by the standard. Since this temperature value depends on the selected load, the specimen geometry, the heating rate, and the selected deflection, the heat deflection temperature is a value with which various materials can be compared. Measurement of the heat deflection temperature of plastics at temperature can be superimposed by internal material stresses. The method is defined in ISO 75 and ASTM D648.

Heat Transfer Methods

  • Since heat is transfered at a defined increase in temperature, heat transfer to the specimen is important in this test method.
  • The classical method uses a heat transfer fluid, usually silicone oil. Test results with a high degree of comparison accuracy are achieved through ample contact between the specimen and the transfer medium.
  • Furthermore, various alternative methods for heat transfer have been established in the standards over the years. One of these alternatives has been technically implemented in the Vicat D model. In this method, the specimen is heated by means of contact heat transfer between two heated plates. This method does not require the use of silicone oil.

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