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Heat Resistance Tests for Plastics

Heat resistance can be determined according to various standardized test methods such as Vicat and the Heat distortion temperature.

Transparent, red plastic parts

Heat resistance is a measure of the thermal endurance of plastic materials. It describes the ability of a test specimen to retain its shape up to a specified temperature under a specified load or to deform only within specified limits at a specified test temperature.

The UL Thermoplastics Testing Center can carry out the following heat resistance tests for you.

Mechanical arm holding red plastic sample piece


This test method is used to assess the dimensional stability of plastics as a function of temperature and determine the Vicat softening temperature.

The Vicat softening temperature (VST) is the temperature at which a standard indenter penetrates 1 mm into the surface of a plastic test specimen under a constant load when the temperature is increased at a uniform rate.

Standards for Vicat
UL 746A, ASTM D1525, ISO 306, or equivalent standards

With Vicat, a distinction is made between:

Method A: 10 N load
Method B: 50 N load

as well as between temperature rise rates of 50 K/h or 120 K/h

A close-up picture of a machine testing plastic.

Heat distortion temperature (HDT)

This material test assesses the dimensional stability of plastics as a function of temperature.

It determines the temperature at which a specified deflection occurs in a specimen positioned on bearings at either end under a defined, concentric load when the temperature is increased at a uniform rate. This value, referred to as the outer fibre strain, is 0.2 % .

Standards for heat distortion temperature
UL 746A, ASTM D648, ISO 75-1/2, or equivalent standards

With heat distortion temperature tests, the standard makes a distinction between three different methods:

HDT, Method A: flexural stress s = 1,8 N/mm² = const.
HDT, Method B: flexural stress s = 0,45 N/mm² = const.
HDT, Method C: flexural stress s = 8,0 N/mm² = const.

Diagram of a ball pressure test

Ball pressure test

This test method is used to determine dimensional stability under stress at elevated temperatures.

It analyses the relationship between the degree of deformation and the temperature when the test specimen is subjected to a constant load.

Standards for ball pressure test
UL 746A, IEC 60695-10-2, or equivalent standards

Results of the ball pressure test method

Test specimen 127 mm x 12,7 mm x 3 mm
Temperature range 23 °C bis 300 °C
Ball diameter 5 mm
Duration of test 60 min
Weight 20 N

Temperature at which the ball produces an indentation measuring 2 mm in diameter.

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