Green sand molds are used to create the space that is filled with molten metal to form the casting upon solidification.
This page covers the No-Bake Sand Molding System:
No-Bake molds are expendable sand molds, similar to typical green sand molds, except they also contain a quick-setting liquid resin and catalyst. Rather than being rammed, the molding sand is poured into the flask and held until the resin solidifies, which occurs at room temperature. This type of molding also produces a better surface finish than other types of sand molds. Because no heat is involved it is called a cold-setting process and is also known as the airset process as molds are left to harden under normal atmospheric conditions. Common flask materials that are used are wood, metal, and plastic.