PEX fitting, whose macromolecules are linear knots, have the biggest disadvantage of poor heat resistance and creep resistance. Therefore, ordinary PEX fitting are not suitable for conveying media with temperature greater than 45 ℃. "Crosslinking" is an important method of polyethylene modification. After crosslinking, the linear macromolecular structure of polyethylene becomes PEX with three-dimensional network structure, which greatly improves the heat resistance and creep resistance of polyethylene. At the same time, its aging resistance, mechanical properties and transparency are significantly improved. The higher the degree of crosslinking, the more obvious the improvement of these properties is. At the same time, it inherits the inherent chemical corrosion resistance and flexibility of polyethylene pipe. There are three types of PEX tubes that have been commercialized.(PEX fitting)