Resin-coated sand takes its name from the resin coating on its surface. This novolac phenolic resin film is solid, distinguishing it from hot-box sand, which is a mixture of liquid resin, an acidic salt catalyst and sand. Outside China, the two types of sand each have their own applications, and their production volumes are nearly equal. In China, resin-coated sand is tending to replace hot-box sand. This trend is not normal, because hot-box sand uses less resin and generally costs less than resin-coated sand. The correct direction for technological development is to select the right core sand for the best technical and economic results. According to the Castings Industry Suppliers Association (CISA) in the United States, among its 52 members, 7 supply resin-coated sand, 10 supply hot-box sand and 8 supply foundry phenolic resins. Including the 8 suppliers of adhesives for shell molds, annual U.S. consumption of foundry phenolic resins is nearly 30,000 t, with resin-coated sand accounting for 60%. All of this coated sand is used to make shell molds and cores, producing nearly 500,000 t of precision castings and thereby generating the highest economic returns.

Jinan Rushun Industry & Trade Co., Ltd. is one of China's largestphenolic resinsuppliers for coated sand, with both a coated-sand plant and a resin plant. It has high output and four distinctive product types: (1) low-odor, low-pollution shell sand; (2) fast-curing shell sand; (3) easily removable shell sand for aluminum castings; and (4) low-nitrogen shell sand for steel castings. Coated sand is primarily used to make shell molds and cores.

Hot-box sand has a wider range of applications than resin-coated sand, and hot-box sands for specific purposes also contain additives to ensure the production of acceptable castings. The four distinctive resin-coated sands discussed in this paper represent technological advances in this field. A common feature of all four is an unprecedented increase in the strength of the resins used. Special high-strength resins can increase room-temperature tensile strength by 30%. Resin-coated sands developed worldwide for making multi-cylinder cylinder-head shell cores use less resin and have very low loss on ignition, so gas evolution does not become a problem in cylinder-head shell-core production.

The quality of phenolic resin is the foundation for developing different types of resin-coated sand. High-quality coated sand requires not only superior resin but also efficient sand-mixing equipment. Only by improving its technical level and expanding production scale can resin-coated sand meet the foundry industry's needs and challenges. At present, China's phenolic resin and resin-coated sand producers are scattered, with low levels of equipment and management. To stabilize product quality and reduce production costs, phenolic resin and coated-sand plants should be appropriately concentrated, overcoming the drawbacks of the current fragmented operations and taking China's resin-coated sand production and shell-mold casting to a new level.