Green Sand Casting

To ensure the quality of castings, dry cores are generally used in sand casting. Depending on the binder used in the cores, they are categorized into clay sand cores, oil sand cores, and resin sand cores.

Clay Sand Core

Simple cores made from clay sand.

Oil Sands Core

Core sand made with drying oils or semi-drying oils as binders is widely used. The low viscosity of the oils ensures good fluidity in the mixed core sand, making it easy to compact during the core-making process. However, the newly formed cores have low strength and typically require a shaped core box for support. They are then baked in an oven at 200-300°C for several hours to harden, as the oil is oxidized by the air. A drawback of this core-making method is that cores are prone to deformation during demolding, handling, and baking, which can reduce the dimensional accuracy of castings; also, the long baking time consumes a significant amount of energy.

Resin sand cores

Various cores made from resin sand. After hardening within the core box, the cores are removed to ensure dimensional tolerances. Depending on the hardening method, the manufacturing of resin sand cores generally falls into three methods: hot-box core making, shell cores, and cold-box core making.

① Hot Core Box Core Making: Developed in the late 1950s. Typically uses furan resin as the core sand binder, with latent hardeners (such as ammonium chloride) added. During core making, the core box is maintained at 200-300°C. After the core sand is injected into the core box, ammonium chloride reacts with free formaldehyde in the resin at higher temperatures to produce acid, causing the mold to harden quickly. The demolding strength is typically achieved in about 10-100 seconds. The hot core box method offers high dimensional accuracy for cores, but the process equipment is complex and expensive, consumes a lot of energy, emits刺激性 gases, and provides poor working conditions for workers.

② The shell cores are produced using the remolding sand heat treatment method, resulting in high strength and good quality sand cores.

③ Cold Box Molding Process: Emerged in the late 1960s. Utilizes urea resin as the core sand binder. When using this method to make cores, the core box is not heated, and blowing ammonia vapor into it for a few seconds hardens the mold. This method is superior to the hot box method in terms of energy, environment, and production efficiency.

In the mid-1970s, the blowing hardening furan resin cold core box method emerged. Its curing mechanism is completely different from the urethane cold core box method, but the process characteristics, such as rapid curing and high core strength, are roughly similar to those of the urethane cold core box method.

Silica Gel Sand Core

Bonded sand cores using water glass as the binder can be categorized into the following types: Water Glass CO2 Method, Ester-hardened Water Glass Self-hardening Method, and Water Glass Methyl Methacrylate Cold Box Method.