The first essential in the process of molding is to select the proper kind of sand for the mold (Fig. 193). In selecting sand, the weight of the casting should be taken into consideration. Mold sand is a mixture of sand, clay, and molasses, or other binding material that aids the sand in retaining its shape under pressure.
Fig. 192. - Pulling Pipe from the Flask. When the iron has cooled sufficiently, the completed pipe is pulled from the flask. The pipe is placed on the skids where the core is cut out and the outside is cleaned.
Fig. 193. - Refining of Sand. Showing foundry derrick for lifting heavy loads of sand, etc., and sieves to separate the fine from the coarse sand.
Sand is said to be sharp when its grains are angular, and dull when they are round. It is termed strong when a body of it manifests a disposition to retain any shape that may be given it, and weak when it tends to fall apart. For light castings the sand should be of fine grain, because less gas is created in light bodies of molten metal than in heavy bodies, and fine, close sand offers more resistance to venting than coarse, open sand. Molds for heavy castings are made from sands of coarse, open-grain texture.