ASTM D648 determines the temperature at which an arbitrary deformation occurs when specimens are subjected to an arbitrary set of testing conditions. ASTM D648 applies to molded and sheet materials available in thicknesses of 3 mm (0.125 in) or greater which are rigid or semirigid at normal temperature.

A bar of rectangular cross section is tested in the sideways position as a simple beam with the load applied at its center to give maximum fiber stresses. The specimen is immersed under load in a heat-transfer medium provided with a means of raising the temperature. The temperature of the medium is measured when the test bar has deflected 0.25 mm (0.010 in). The temperature is recorded as the deflection temperature under flexural load of the test specimen.


  1. Begin preparing your test samples from stock material
  2. Then measure the width and depth of the samples with a micrometer at various points to determine the average width and depth
  3. Determine the amount of applied force necessary to produce the specified fiber stress in each specimen
  4. Position the test sample perpendicular of the molding flow, making sure they are properly aligned on the supports
  5. Position the thermometer bulb within 10 mm (0.039 in) to the test sample but not touching
  6. Make sure that the bath temperature is a ambient temperature to start
  7. Apply the loaded rod to the test sample and carefully place the assembly into the bath

By means of our plastics testing expertise and modular product design, we will help find the solution that is right for you. Give our test engineers a call today for help configuring the best test machine and accessories according to ASTM D648.

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