The HDT/VSP instrument is used for determining the deflection temperature limit for specific application and comparing the heat softening characteristics of thermoplastic material at a constant load in oil upto 300 Deg. C^o.c

Plastic materials being heat sensitive in nature has a lot of bearing on its thermal environment for it's performance. Though their behaviour in thermal conditions is very complex some simple yeild tests are used to predict it's behaviour and performance in a given condition. "VICAT SOFTENING POINT" or "HEAT DISTORTION TEMPERATURE" is one of such yeild points very commonly employed with such demanding applications. Vicat test consist of penetrating a unit size indentor in a plastic surface with predefind load under constant rate of environmental temperaturerise. Heat distorsion test, as it is sometimes called, consistes of deflecting a simply supported beam of test material under similar heating conditions & with a predefined stress application.

HDT
Test for deflection temperature of plastics under flexural load. Heat distoration temp. is the temp. At which a sample deflacts by 0.1 inch. (2.5 mm). Standart test methods: ASTM D648, BS 2782 method 102, ISO R75.

VST
Vicat softening point is the temperature at which the needle penetrates 1 mm into the sample. Standard test methods : ASTM D1525, BS 2782 method 102D, ISO R306.