GB/T 1634.1-2019 is a national standard for the general test method for determining the deflection temperature of plastics under load. As the first part of the GB/T 1634 series of standards, it specifies the method for determining the deflection temperature of plastics under three-point bending stress, including requirements for different types of test specimens and constant test loads. It is applicable for evaluating the relative heat deformation resistance of materials under load and heating conditions. The testing principle involves subjecting the specimen to a constant load and heating it at a uniform rate until the standard deflection is reached, at which point the corresponding temperature is recorded. Related testing instruments include heat deflection testers and universal material testing machines (equipped with high-temperature environmental chambers and three-point bending fixtures), paired with high-precision temperature control systems, laser displacement sensors, or direct-reading micrometers to ensure test accuracy.