380 APPLIED SCIENCE 



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. 



422. Branches of Molding. There are three branches 

 of molding (1) green-sand, (2) dry-sand, and (3) loam 

 molding. 



Green-sand molding involves the making of castings in molds 

 that are composed entirely of damp sand, or that have their 

 surfaces "skin dried," that is, dried by building a fire in the 

 mold to harden its surface without baking its entire thickness. 



In dry-sand molding, the sand is damp when the metal is 

 poured, after which the mold is dried in an oven, or other- 

 wise, so as to remove all moisture and leave the body of the 

 mold dry and firm. In other words, the sand is drier than 

 green sand. 



In loam-sand molding, castings are made in molds con- 

 structed with skeletons of patterns. A mixture of loamy sand 

 and other material is used to form the surface of the mold 

 and to form its outer and inner supports. This class of work, 

 like dry-sand molding, requires drying before it is ready to 

 receive the melted metal. 



423. Green-Sand Molding. The practice of some shops 

 embraces all three kinds of molding, but most foundries 

 make only green-sand molds. These involve, however, 

 more risk in making medium-sized and large castings. In 

 many cases inexperienced men may be employed for making 

 dry-sand molds, but it is seldom wise to trust other than 

 skilled workmen with the construction of green-sand molds, 



