The heat deflection temperature of plastic materials is the temperature at which the material deflects by a specified amount under the action of a specified stress. It is determined by supporting a rectangular test specimen over two cylindrical edged blocks and pressing it at the center of the supported span through a cylindrical edged loading tip. The test specimen, along with the supports and the loading frame, is kept in a high temperature bath whose temperature is slowly raised. The temperature at which the test specimen deflects through a predetermined distance is the heat deflection temperature of the material under test.