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HDT (DTUL)/Vicat 

Heat Deflection Temperature (HDT) is often now referred to as "DTUL" (Deflection Temperature Under Load). It is the temperature at which a given amount of deflection occurs, usually 0.25 mm (0.010 in), at a given loading of either 0.45 MPa (66 psi) or 1.8 MPa (264 psi). The sample is placed in a fixture which supports it at a 102 mm (4.0 in) span. Weight is concentrated at the center of the span on top of the sample to supply the stress. The fixture is lowered into a liquid bath (usually silicone fluid). The temperature is raised at 2oC per minute. When the deflection at the center of the span reaches 0.25 mm, the temperature is recorded.

 There are several key factors which can significantly affect the results of this test. Sample thickness for this test can vary from 3.2 mm (0.125 in) to 12.7 mm (0.500 in). The weight is increased with thicker samples which makes the stress consistent with the thinner samples. However, while the temperature is being raised there is a longer time lag for the center of a thicker sample to reach the temperature at which it will deflect. The temperature of the medium continues to rise. When the required deflection is reached and the reading is taken it is higher than what would be obtained for a thinner sample.

 Stress is another important influence on HDT (DTUL). Samples with high levels of molded in stress will exhibit lower values. During injection molding, the molecules are stressed going through the gate and by cavity filling dynamics. The polymer often solidifies before these molecules can move back into a low stress position. The resultant molded in or "frozen in" stress is significant. When the samples are heated in the thermal bath, the energy that has been stored in the material in the form of frozen in stress increases the load on the sample causing it to deflect at a lower temperature.

Compression molded samples have very low molded in stress because they are produced without significant material flow. These samples will exhibit significantly higher HDT (DTUL) values. Annealing of samples before testing reduces the molded in stress of the parts by raising the temperature to a point where localized molecular movement can occur. This relieves the majority of the molded in stress from injection molding and will result in a higher HDT (DTUL) value.

 When using HDT (DTUL) to compare thermal performance of resins, sample thickness, method of fabrication, processing conditions, load, and post molding conditioning must all be considered. For these reasons it is often difficult to draw conclusions between HDT (DTUL) data and end use performance, or to compare various polymers.


Deflection Temperature of Plastics Under Flexural Load (ASTM D648)


To determine the temperature at which an arbitrary deformation occurs when specimens are subjected to an arbitrary set of testing conditions.

 This test is particularly suited to control and development work.  Data obtained by this method may not be used to predict the behavior of plastic materials at elevated temperatures except in applications in which the factors of time, temperature, method of loading, and fiber stress are similar to those specified in the test.  The data are not intended for use in design or predicting endurance at elevated temperatures.


A bar of rectangular cross section is tested as a simple beam with the load applied at its center to give maximum fiber stresses of 66 psi or 264 psi.  The specimen is immersed under load in an oil bath that is heated at 2 ± 0.2oC/min.  The temperature is recorded when the test bar has deflected 0.010 inch.  This is reported as the deflection temperature under flexural load of the test specimen.

 Vicat Softening Temperature of Plastics (ASTM D1525)


To determine the temperature at which a specified needle penetration occurs when specimens are subjected to specified test conditions.  This method is useful for many thermoplastic materials.  It is not recommended for materials having a wide softening range (e.g. non-rigid PVC).

 Data obtained by this test method may be used to compare the heat softening qualities of thermoplastics.  This test is useful in quality control, development, and characterization of materials.


A flat ended needle loaded with a specified mass is placed in direct contact with a test specimen.  The specimen and needle are heated in an oil bath at either of two permissible rates:

             Rate A = 50 ± 5oC/hr

            Rate B = 120 ± 12oC/hr

 The temperature at which the needle has penetrated a depth of 1 mm is the Vicat softening temperature.  At least two specimens are used to test each sample.