Heat Deflection Temperature ASTM D648, ISO 75

Heat Deflection Temperature ASTM D648, ISO 75

ASTM D648, ISO 75 is used for the determination of Deflection Temperature Under Load (DTUL or HDT) data for polymer datasheets and material specifications. This test helps in identifying suitable material for design application that can sustain light loads at high temperatures.


    Heat Deflection Temperature (HDT) test by ASTM D648 is performed to determine the short-term heat resistance of polymeric materials. To find a suitable material for design application, it is necessary to identify materials that lose rigidity over a narrow temperature range and materials that can sustain light loads at high temperatures. Therefore, The heat deflection temperature is the temperature value at which a standard test bar of that material deflects a specified distance under a load.

    Test Procedure:

    A deflection measuring device is employed for deflection temperature under load test by ASTM D648. Specimen bars of the test material are placed under the measuring device. Each bar is applied with a load of 0.45 MPa or 1.80 MPa. The specimen bars are then let down into a silicone oil bath, where the temperature is raised at 2° C per minute. The specimens are lowered down until they deflect 0.25 mm for ASTM. The deflection value is 0.32 mm for ISO flatwise and 0.34 mm for ISO edgewise.

    Specimen size: 

    Specimen bar size for ASTM is 5 inches × ½ inch × ¼ inch. As per ISO, edgewise testing is done using a bar having measurements 120 mm × 10 mm × 4 mm. For ISO flatwise testing, a bar measuring 80 mm × 10 mm × 4 mm is required.


    The temperature value is recorded at the specified load and deflection.


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