In the process of using coated sand, common defects, main causes and solutions are as below:
1. The surface of the type (core) is loose; The pressure of the core is too high or too low; The exhaust of the mold is not smooth; The mold runs sand due to the large gap of the box surface; The fluidity of the coated sand is poor or the gas permeability is poor. Use reasonable shooting pressure to improve the exhaust system to prevent helium; Use the material with small deformation as the core box; Choose coated sand with good fluidity and permeability.
2. The design of the shelling mold is unreasonable, the temperature of the core box is not uniform, it leads to unshelled because of low strength of the low temperature part; The melting point of the coated sand is low, the curing speed is slow, and the heat intensity is low. Improve the structure of the mold to make the temperature distribution uniform; Use the resin with high melting point, fast curing speed and high heat strength.
3. The casting venting core is not exhausted; The resin sand has a large amount of gas or the gas velocity is not suitable; The sand core is not completely cured. Improve the exhaust system and improve the exhaust effect; Use the original sand with high concentration or coarseness; Use the low-gas coated sand process.
4. The core has low local strength or looseness; The thickness of the crust is thin. Improve the structure of the mold to make the temperature distribution uniform; Select the resin with high melting point, fast curing speed and high heat intensity; Adjust the shooting pressure; Improve the exhaust system.
5. The SiO2 content of the casting sand is low; The core surface is not dense. Adjust the shooting pressure, improve the core box exhaust effect to make the sand core surface more dense; Use high temperature coated sand or zircon sand coated sand.
6. Type (core) deformation, fracture mold heat unevenness or core wall thickness difference, result in inconsistent shrinkage during cooling; Using the resin with a large cure shrinkage; Core fork deformation or sand core storage uneven; High temperature properties of coated sand is poor; Pouring pressure is too large; Improve the mold structure to make the temperature distribution uniform; Use the forming tray to store the sand core; Use the resin with small cure shrinkage; use high temperature and low expansion coated sand; Improve the pouring system (pressureless).
7. The inside of the casting is loosened, the resin in the coated sand is burned at a high temperature to generate heat, which slows down the solidification speed of the molten iron and causes shrinkage. The chilled coated sand is used; The inner cold iron is placed in the shell core.
8. The surface of the casting is defective. The lustrous carbon generated by the phenolic resin at high temperature floats on the surface of the molten iron, and the surface of the casting is wrinkled when solidified. Add about 2% iron oxide powder, use raw sand with high thermal conductivity; Brush surface coating on the shell core; Add special auxiliary materials in the coated sand.