1. Thermal expansion of materials: During the stretching process, materials will be subjected to tension and friction to generate heat, leading to thermal expansion of materials. The thermal expansion coefficient of the material is large, so the inner diameter of the material becomes smaller after the wire drawing die is heated up.

2. Die wear: The wire drawing die will be rubbed and squeezed by materials during use, resulting in die wear. The die is badly worn, so the inner diameter of the die will become smaller after the wire drawing die is heated up.