This standard specifies four test methods for determining the Vicat softening temperature of thermoplastic materials, where the measured temperature reflects the characteristic point at which the material begins to soften rapidly. The core principle involves immersing the specimen in a liquid heating medium, applying a specified constant load (e.g., 10 N or 50 N) under a uniform heating rate, and recording the temperature at which a standard indenter penetrates the specimen surface to a specified depth. This temperature is defined as the Vicat softening temperature. The specialized equipment for conducting this test is the Vicat softening temperature tester (also known as a heat deflection and Vicat temperature tester), which precisely controls the heating rate and applied load while automatically detecting indenter displacement and recording the corresponding temperature. Additionally, certain thermomechanical analyzers with comprehensive testing capabilities can also be used for this determination. This method provides a standardized testing basis for evaluating the heat resistance of thermoplastic materials.