480 ries. FOUNDRY SANDS [Ch. 26 



of the brittleness of a sand mixture. The deformation is expressed in 

 thousandths of an inch per inch. By taking the product of deforma- 

 tion and green compressive strength and multiplying this by 1,000, 

 the STN, or sand toughness number, which is an indication of the 

 workability of the sand, is obtained. By properly controlling the de- 

 formation, the foundryman may avoid various troubles and even cast- 

 ing defects. 



EFFECT OF ELEVATED TEMPERATURE 



Foundry sands react to different metals in different ways at elevated 

 temperatures. A number of procedures have been used to test the 

 sand under these conditions, although no tests have yet been stand- 

 ardized. 



Hot compression test. A specimen of the sand 1% by 2 inches is 

 placed in a special furnace and heated to various temperatures ; then it 

 is subjected to increasing pressure until it collapses. Some clay bonds, 

 as, for example, bentonite, give a higher hot strength than fire clay. 

 The hot compressive strength also tends to increase as the temperature 

 rises to 1,800° or 2,000° F., after which it decreases. No strict rules 

 can be laid down for the hot compression test, and it is necessary for 

 the foundryman to determine what hot strength gives the best results 

 with a particular casting, and adjust the mixture accordingly. 



Expansion and contraction. Sand grains expand when heated, 

 whereas clay contracts. If a bonded sand is heated, it expands at first, 

 but, with continued rise in temperature, it begins to contract. If this 

 expansion of the sand in the mold is too great, it causes defects in 

 the casting. Excessive expansion is undesirable, and various means 

 may be taken to reduce it (Dietert et al., 1939; Dunbeck, 1946). 



Core collapsibility . The shrinkage of solidifying metal around a 

 core applies to it pressure which, if resisted by the core, develops tears 

 or strains in the casting. It is therefore necessary to select a core 

 mixture which, when hot, yields to the metal or collapses. This mix- 

 ture is selected by placing a test specimen in the furnace and sub- 

 jecting it to a predetermined load, often 50 pounds, and noting the 

 time that the core can withstand this without collapsing. 



Heat shock. If a test piece, when placed in a hot furnace at the 

 casting temperature of the metal with which it is to be used, spalls or 

 cracks, the foundryman may feel that the sand will not work well for 

 large castings. Consequently samples are sometimes tested for heat 

 shock. 



