Flatness of cast steel coated sand: mainly related to poor high temperature resistance of coated sand. Some high temperature resistant additives can be added. This additional solid state sintering at high temperature makes the casting smooth. The surface of the sand particles is covered with solid resin film sand or core sand. There are two coating methods: cold method and hot method: the cold method uses ethanol to dissolve the resin and adds atropine in the sand mixing process, so that the two are combined on the sand surface, and the ethanol volatilizes to obtain the coated sand; The sand is preheated to a certain temperature by thermal method, the resin is transformed into a melting temperature, the resin is introduced into the sand surface by stirring, and the utopine aqueous solution and coolant are added for cooling, crushing and screening. For cast steel and cast iron.