This is a fundamental standard for testing the heat resistance of plastics, which describes a general method for determining the deflection temperature of plastics under load in three-point bending. This method is an important means of evaluating the relative heat resistance levels of different types of plastics under specified loads and heating rates, and its results are significant for material selection and quality control. The core equipment of the experiment is the deflection temperature under load tester (also known as the heat distortion tester), and the standard specifies the preferred specimen dimensions for testing. This standard serves as a general guideline, and its subsequent parts (such as Part 2 and Part 3) will further specify the methods for specific types of materials and applications.