For the preparation of magnesia powder 30-50%, pearlescent powder 20-40%, mica powder 15-30%, silica sol 3-9%, white emulsion 1-5%, polyacrylamide 0.1-0.8%, sodium carboxymethylcellulose 0.1-0.8%, adsorbent 1-6%, flame retardant 0.3-2%, the adsorbent, sodium carboxymethylcellulose and polyacrylamide are respectively added with water to form a solution, and then mixed with aggregate and other components Mix it. The coating of the invention can effectively adsorb the liquid products produced by the pyrolysis of the foam plastic pattern, greatly reduce the porosity of the castings, and form a reducing protective atmosphere layer between the mold magnesium liquid interface so as to form a protective film on the surface of the magnesium alloy, prevent oxidation, prevent the combustion of magnesium alloy in the mold when pouring, and facilitate the obtaining of the magnesium alloy casting with smooth surface and wrinkle free surface. Negative pressure can help the pyrolysis products to be discharged from the mold and reduce the wrinkle defects. However, it should be noted that the negative pressure should not be too low, otherwise it is easy to cause penetration and sand sticking defects. The effect of pouring temperature on wrinkling defects ofcastings. Generally speaking, increasing the pouring temperature is conducive to the gasification of pyrolysis products and the discharge of the products, thus reducing the wrinkle defects. For high chromium cast iron, the pouring temperature is higher than that of gray cast iron and nodular iron, and wrinkle defects can be basically eliminated.
The top pouring system is beneficial to reduce the surface wrinkle defects, but it is easy to cause internal carbon black inclusions. The bottom pouring pouring system can realize the stable rise of liquid metal and the gasification of pattern layer by layer without carbon black inclusion inside; however, more pyrolysis products tend to gather on the upper surface of the casting, and the temperature of the upper liquid metal is the lowest, which is not conducive to the gasification of pyrolysis products and easy to form severe wrinkle defects. Therefore, it is necessary to set a slag collection riser at the upper part to try to discharge the pyrolysis products out of the casting. Generally, the higher castings use step gating system, which can not only achieve stable filling, reduce carbon black inclusion defects, but also avoid wrinkle defects. Top pouring system can only be used for small parts with small height