The mechanism of fiberglass failure in the mold is mainly that the temperature of the mold is much lower than that of the melt. After the melt flows into the cavity, a frozen layer is formed on the inner wall immediately, and with the continuous cooling of the melt, the frozen layer is formed. The thickness of the fiberglass continues to increase, so that the intermediate free-flowing layer becomes smaller and smaller, and part of the glass fiber in the melt adheres to the frozen layer and the other end still flows with the melt, thus forming a large shear force on the fiberglass resulting in breakage. The thickness of the frozen layer or the size of the free-flowing layer will directly affect the flow of the melt and the magnitude of the shear force, which in turn affects the degree of damage to the fiberglass. The thickness of the frozen layer first increases and then decreases with the distance from the gate. Only in the middle, the frozen layer thickness increases with time. So at the end of the cavity, the fiber length will return to a longer level.