Determination of Vicat Softening Temperature and Heat Deflection Temperature

Thermoplastics do not have a definite melting-point which precisely marks the transition from solid to fluid, but rather undergo a slow softening as temperature increases. 

Vicat Softening Temperature was therefore introduced to measurement technology as a substitute value for melting-point. It describes the temperature at which a circular indentor with a cross-section of 1 mm² under a standardized loading of 10 N or 50 N penetrates exactly 1 mm into the specimen. The Vicat Softening Temperature is standardized in ISO 306 and ASTM D 1525.

Another measureand used is the Heat Deflection Temperature, for which a prismatic specimen is subjected to flexural loading and the temperature increased at a specified rate. The heat deflection temperature is achieved when the specimen deflects to a distance specified in the Standards. As this temperature naturally depends on the load applied, the specimen geometry, the heating rate and the selected deflection, the heat deflection temperature represents a value which can, for example, be used to compare different materials with each other. Heat deflection temperature measurement of plastics under temperature can be affected by internal material stresses. The method is specified in ISO 75 and ASTM D 648.