ASTM D648, 1SO 75 Heat Deflection Temperature of Plastics Under Flexural Load in the Edgewise Position

Introduction to ASTM D648

ASTM D648 is a test method designed to measure the deflection temperature of plastics. The applications of the plastics are specified as per the results obtained. The fiber stress, method of loading, factors of time, and humidity contribute to the determination of the deflection temperature of plastics. The results obtained are specially used for quality control and specification purposes. The deflection temperature is a measure of a polymer’s ability to bear a given load at elevated temperatures. The deflection temperature is the temperature at which a test bar, loaded to the specified bending stress, deflects by 0.010 inch or 0.25 mm. The deflection temperature is also known as the ‘heat deflection temperature’, ‘deflection temperature under load’ (DTUL), or ‘heat distortion temperature’ (HDT). ASTM D648 test method is the common test method whereas ISO 75 is the analogous method. The test using a 1.8 MPa load is performed under ISO 75 Method A while the test using a 0.46 MPa load is performed under ISO 75 Method B.

ASTM D648 test method is generally applicable to both semi-rigid and rigid materials. The method is important for making recommendations about the use of the material in developmental work and provides information about short-term heat resistance. It also differentiates between materials that are able to sustain loads at high temperatures and those that lose rigidity over a narrow temperature range. 

ASTM D648 Test Method

This test method proceeds by testing the rectangular bar in the edgewise position as a simple beam with the load applied at its center to give maximum fiber stresses. The specimen is immersed under the load in a heat transfer medium at a raised temperature. The test bar begins to deflect by 0.25 mm after some time has passed. The temperature of the medium at this point is recorded as the deflection temperature under the flexural load of the test specimen. 

ASTM D648 Specimens

ASTM D648 Results

This test method evaluates the deflection properties of plastics along with the determination of the temperature at the specified load and deflection. 

Tests related to ASTM D648
ISO 75 is similar to ASTM D648. As per ISO 75, the bars are placed under the deflection measuring device. A load of 0.45 MPa or 1.80 MPa is placed on each specimen. The specimens are then lowered into a silicone oil bath where the temperature is raised at 2 °C per minute until they deflect 0.32 mm for ISO flat-wise, and 0.34 mm for ISO edgewise. ISO edgewise testing uses a bar 120 mm x 10 mm x 4 mm. ISO flat-wise testing uses a bar 80 mm x 10 mm x 4 mm.

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