Ch. 26] PROPERTIES OF FOUNDRY SANDS 479 



with no clay or other bonding substance; (2) green permeability, or 

 permeability of a molded mass of sand in a moist condition; (3) dry 

 permeability, or permeability of a molded mass of sand which has been 

 dried at 105° to 110° C. and cooled in a desiccator to room tempera- 

 ture; (4) baked permeability , or permeability of a molded mass of sand 

 baked at a temperature above 100° C. 



Green compressive strength. To be effective foundry sands must 

 possess adequate strength. In order to test the strength, the green 

 compressive strength is determined immediately after the sand has 

 been mixed. For details of this test see American Foundrymen's As- 

 sociation (1944). The green compressive strength generally increases 

 with the amount of clay bond, but different clays have different effects. 

 The compressive strength increases with the addition of moisture 

 to an optimum condition and then decreases with the addition of more 

 water. According to Grim and Cuthbert (1945) the strength increases 

 from 12 pounds per square inch for a moisture content of 1.0 percent 

 to 18 pounds per square inch for a moisture content of 2.0 percent and 

 then decreases to 9 pounds per square inch for a moisture content of 

 4.0 percent. The strength also increases with amount of tamping or 

 ramming during preparation of the sample. Work by Davies and Rees 

 (1944) on synthetic sands suggests that round grains give the sand 

 greater strength than angular grains. 



Dry compressive strength. Sands change in strength as they dry. 

 A measure of strength called the dry compressive strength is taken 

 after the sand has dried for 2 hours at a temperature of not less than 

 105° C. The dry compressive strength varies with (1) amount of 

 moisture in the green sand, (2) extent of mixing, (3) number of rams, 

 (4) content of clay, water, or other bonding material, and (5) grain 

 size. 



Flowability. The American Foundrymen's Society defines flowabil- 

 ity as "the property of a foundry sand mixture which enables it to fill 

 pattern recesses and move in any direction against pattern surfaces 

 under pressure." It is, as can be readily understood, an important 

 property, but as yet there is no standard test for it, although several 

 have been suggested (Chadwick, 1940; Kyle, 1940; Lissell and Ash, 

 1942). 



Air set strength. Another property is the air set strength, which is 

 the strength the sand develops when it is allowed to stand in the air, 

 even though all its moisture has not evaporated (Grim and Cuthbert, 

 1945). 



Deformation. The deformation is defined as the change in linear 

 dimensions of a sand mixture in response to stress. It is a measure 



