The Heat Distortion Temperature Test of Plastics (HDT) is a standard test (ASTM D648 and ISO 75) used to assess the temperature at which a polymer, plastic or plastic composite specimen is subjected to a given flexural load (fiber strain). ) is deformed. The heat distortion temperature is also known as the 'deflection temperature under load' (DTUL) or 'heat deflection temperature' (HDT).
The properties of plastics, like all materials, depend on temperature. This is of particular importance for mechanically stressed parts, which are exposed to high temperature use. As a result the heat distortion temperature property of plastic materials is of paramount importance and has become an essential tool for product design, engineering and product manufacturing, making the heat deformation temperature test of plastics important.
During the test a plastic sample with given dimensions is loaded into a heat distortion temperature test apparatus, aligned in a plane or edge direction, in a three-point bend style test. The direction of sampling, depending on the standard used, ASTM or ISO, is a very important part of the overall test as it was found to significantly affect the result. Thus, special care should be taken to load the sample in the correct position, in accordance with the relevant standard.
The fiber tension applied to the test is either 0.455 MPa or 1.82 MPa, and the temperature is increased at a rate of 2 °C/min until the specimen is deflected by 0.25 mm (for ASTM) Go. The resulting thermal deflection temperature of the tested plastic will be the temperature at which the sample is deflected by 0.25 mm. Due to the vastly different standards (ASTM and ISO) it is necessary to mention not only the HDT value and applied stress, but also the standard used.