Bonding agents are molding materials used to bind loose foundry sand particles together, thereby transforming them into molding sand or core sand. When mixed with sand grains, the bonding agent coats the surface of each grain, forming an adhesive film that causes the grains to adhere to one another; this imparts sufficient strength to the sand molds and cores to prevent deformation or breakage during handling, assembly, and casting operations. The primary material used in the clay molds of ancient Chinese foundry practice (historically referred to as *taofan*) was clay, which possesses strong bonding capabilities. As technology advanced, significant quantities of sand grains were incorporated into these clay molds; eventually, sand became the primary constituent material, with clay assuming the role of the bonding agent. Clay remains in widespread use as a bonding agent to this day. Subsequently, various inorganic and organic bonding agents emerged-including vegetable oils, rosin, dextrin, water glass, and synthetic resins. In 1943, J. Croning of Germany invented a process for producing thin-shell sand molds using phenolic resin as the bonding agent.
In 1947, L. Petrzela of Czechoslovakia utilized water glass as a bonding agent for molding sand and introduced carbon dioxide (CO2) gas to induce hardening, thereby producing sand molds and cores. The application of these two bonding agents pioneered a new pathway for the chemical hardening of sand molds and cores. Chemical hardening involves adding a small quantity of a hardener to specific organic or inorganic bonding agents; through physicochemical interactions between these components, the sand molds and cores are caused to harden rapidly within a short timeframe. Castings produced using chemically hardened sand molds exhibit significantly improved dimensional accuracy, surface finish, and production efficiency; consequently, this technology was quickly adopted for widespread use. From the late 1950s onward, countries around the world progressively adopted furan resin bonding agents; using this method, cores could be fully hardened within just one to two minutes when formed inside a heated core box.
