HIGH LEADED TIN BRONZE

Some of the principal problems experienced in casting high-leaded tin bronzes are caused by low-melting constituents composed of the tin and lead in the alloy. At normal pouring temperatures, these alloys are so fluid that they may penetrate the mold. Attention should thus be given to the type of sand used and to the use of a mold or core wash that will resist metal penetration.
Leaded-tin bronzes may pick up gas which can cause casting defects, including one known as "mushrooming" or "purging". Gas picked up in the me(ting may accumulate in the last portion of the casting to freeze. building up sufficient pressure to blow the liquid metal up through the sprue or riser. The best possible melting practices must be applied to reduce gas pickup in melting. Liberal use of phosphor copper is also recommended. Additions of 15% phosphor copper up to six or eight ounces per hundred pounds of metal will often help insure sound castings.
Bushings or bearings cast in these alloys often have large cores. The gases given off by the cores may dissolve in the metal and show up as holes during machining. To prevent this from happening, the cores should be as permeable as possible. In many cases, hollow cores are desirable. Very hard cores should be avoided beC4use they usually contain unburnt oil, and also increase the possibility of hot-tearing around the core when the meta~ shrinks. Cores made by the C02 process or by the shell process may reduce gas from this particular source.
Application of the practices described above will help to control the subsurface porosity occasionally found when high-leaded tin bronze castings are machined.