The aluminum shell is produced and cast under high temperature. The production speed of aluminum shell is too fast, and the cooling strength is relatively large, resulting in some elements in the alloy can not be converted into the required elements within the specified time. Because this material has needle like structure, high hardness and poor plasticity. Moreover, the tensile strength of this manufacturing process is very low, and not only yellow rust but also cracks will appear at high temperature. The aluminum shell produced in this way will have granular burrs. If the burrs are not removed, it will have a very bad feel. Tadpole like tails are often accompanied by such particles, which can be observed under a microscope. Moreover, the color of such impurities is gray, containing a lot of iron elements.