Process Flow Diagram
Coated shell casting is a precision casting technology that uses sand particles (coated sand) coated with thermosetting resin as the core material and is hardened by heat to form a thin shell mold (6-12mm). The process includes four steps: sand mixing, shell making, curing and pouring, which can achieve a dimensional accuracy of ±0.3mm and a surface finish of Ra6.3μm, especially for the production of complex thin-walled parts (minimum wall thickness of 1.5mm).
Sand Mixing
The intelligent sand mixing system has evolved through four stages of development for coating sand: dry mixing, cold coating, warm coating, and hot coating. Currently, the hot coating method is predominantly utilized in industrial production.
The hot coating process specifically involves heating the raw sand to temperatures ranging from 180 to 280°C. Subsequently, resin, uranyl nitrate solution, and calcium stearate are added sequentially, followed by thorough mixing and stirring. After the mixture has been stirred, it undergoes cooling, crushing, and sieving to produce the final coated sand. Given the varying performance requirements for coated sand across different applications, the specific formulas and pouring process details also differ accordingly.
▸ Sand temperature monitoring (180-280℃±5℃)
▸ Resin ratio (2.5%~4.0% automatic metering)
▸ Urotropine activation (hexamethylenetetramine 10%~15%)
▸ Calcium stearate lubrication (0.3%~0.8% anti-sand)
Shell Making
Cover the prepared coated sand over a metal mold heated to 180-280°C (metal molds are typically heated to around 300°C). Once the coated sand is heated, the resin on its surface gradually melts. Under the catalysis of methylene released from urea decomposition, the molten resin rapidly solidifies from a linear structure into a stable three-dimensional shape, forming a hard thin shell with a thickness usually between 6 and 12 millimeters. In actual production, efficient ladle casting techniques are commonly used for manufacturing thin-shell molds, while flexible blow molding technology is preferred for producing thin-shell cores.
▸ Metal mold temperature control system: 300℃±10℃ intelligent constant temperature
▸ Shell thickness control technology: 6-12mm adjustable sand scraping
Curing Stage
Further heating is applied to the initially formed thin shell to ensure it achieves sufficient strength and stiffness. Subsequently, the upper and lower shells are securely fastened together using clamps or resin bonding, forming a complete mold. This mold can be ready for metal casting without the need for a sand box.
▸ Rapid curing scheme: methylene crosslinking reaction <30s
Pouring System
The pouring system introduces the qualified molten metal into the mold. Once the metal cools and solidifies, the mold is opened and the casting is removed. Subsequently, residual sand, oxide scale, and other impurities on the casting's surface are eliminated. This completes the entire casting production process.
▸ The density of the casting is increased by 20%
▸ Pore defect rate <0.5%
▸ The minimum cast hole diameter is φ3mm
