The sheet used for forming, whether it is single-layer or multi-layer composite, must have the following properties: 1. Plastic memory, that is, when the softened sheet is stretched, it has both the tendency to shrink and resist the tensile force, and the A tendency to stretch evenly. This feature allows an already formed article to return to its original flat sheet shape if it is reheated to its original forming temperature. This characteristic has an important influence on the stretching of the forming process. 2. Thermal stretching, that is, the sheet can be stretched when heated, which has a great influence on the shape and quality of the product. Some can stretch 15% to 20%, while some can even stretch to 500% to 600%. 3. Thermal strength, that is, as long as the heated and softened sheet is subjected to a little pressure, a clear outline will be formed on the mold. On the contrary, if too much pressure is required to form, and the pressure difference provided by vacuum forming is limited, it is difficult to display some fine patterns. Fourth, the forming temperature, that is, the forming sheet needs to have a suitable processing temperature range. Not only is it easy to form at its softening temperature when heated, but also has a certain distance from its melting temperature, and the forming temperature range is wide; it cannot only be formed in a small specific temperature range. When the temperature is high or low, the forming is easy to tear. , collapse, etc.