478 bibs. FOUNDRY SANDS [Ch. 26 



fire clay, bentonite, and illite. Various oils and organic substances 

 are also used. The sand mixture must have sufficient moisture added 

 to it to cause the particles to cohere.) 



(2) Sufficient refractoriness to resist the heat of the molten metal. 



(3) Sufficient strength to resist the pressure of the metal. 



(4) Sufficient permeability to permit water vapor and gases to es- 

 cape outward from the mold instead of being forced into the molten 

 metal. 



(5) Proper texture so that the mold surface will be sufficiently 

 smooth to produce a smooth surface on the casting and not develop 

 surface defects. 



These are the main requisites, and the proper mixture is obtained 

 by using sand of appropriate grain size, proper amount of bond, and 

 correct amount of moisture. The various properties which control 

 the conditions above mentioned can be determined by special tests, 

 most of which have been standardized by the American Foundrymen's 

 Society (formerly the American Foundrymen's Association). A 

 number of these tests are made on the sand at room temperature; 

 others are performed on the material at elevated temperatures. After 

 a sand mixture of the desired properties is found, an effort is made 

 to keep these properties fairly constant. The properties are briefly 

 described below. 



Permeability. The sand must be sufficiently permeable to allow 

 volatile matter to pass through it as the casting cools. Permeability 

 should not be confused with porosity, as it often is. Permeability is 

 an important property, because the hot metal in contact with the sand 

 generates a mixture of water vapor and gases from organic compounds. 

 These gases must be able to pass outward through the sand; otherwise 

 they may be forced into the molten metal and cause defects. 



Fine-grained sand, other things being equal, has less permeability 

 than coarse-grained sand. The quantity and character of the clay also 

 exert an effect, as does the amount of moisture. The permeability in- 

 creases with the amount of moisture up to what is known as the op- 

 timum, after which it decreases. 



Permeability is determined by measuring the rate at which air flows 

 through a standard AFS specimen under standard pressure. This 

 standard specimen is 2 inches in diameter, 2 inches high, and is com- 

 pacted or rammed under standard conditions (American Foundry- 

 men's Association, 1944). 



The permeability of a sand is determined under several conditions: 

 (1) base permeability, or permeability of the packed, dry sand grains 



