Plastics and thermoplastics materials are temperature sensitive. The HDT and Vicat tests consist of determining the temperature at which a stressed sample undergoes deflection: in the HDT test the sample undergoes flexion, while in the Vicat test the sample is penetrated by a point.
WHY IS HDT VICAT TESTING IMPORTANT?
Plastic materials are very sensitive to temperature changes. Unlike metals, polymers are extremely sensitive to changes in temperature and many factors contribute to affect their thermal behavior, which is a rather complex property. The thermal properties of plastic materials are equally as important as the mechanical properties. Selecting a suitable plastic is a big challenge for Designers and material selectors of plastic products for elevated temperature performance.
The HDT (ISO 75 andASTM D648) involves a sample being stressed by flexion in correspondence to a given initial force applied, while immersed in a temperature-controlled bath whose temperature is increased at a constant rate. The temperature at which the test specimen undergoes deflection of an established value corresponds to the result of the analysis.
In the case of the VICAT (ISO 306 and ASTM D1525) test, a penetrating element with a circular section (needle), loaded with a pre-defined weight, is rested on the sample. The prepared test specimen is then immersed in a temperature-controlled bath, whose temperature is increased at a constant rate. The temperature at which the needle penetrates the sample by a set amount corresponds to the result of the analysis.