Study of porosity and voids in dental gold castings.

The problem of voids and porosity has long plagued the dental technician. Many times an otherwise acceptable casting has had to be discarded because voids or porosities have developed within the casting. Because of numerous variables a technician often does not know the cause of the casting failure. Many investigators have studied variables that may affect the occurrence of voids and porosities.'-5 Asgar6 has conducted a study on the effects of sprue gauge, length, direction, shape, number of sprues, and flaring. He also studied the pressure of the casting machine and the amount of gold used when casting. He found that all of these variables have some effect on the occurrence of voids and porosities. Tucillo and Nielsen7 have reported that short, thick sprues tend to minimize local porosity but tend to form surface porosities. They found that the reverse was true with long, thin sprues. Brumfield8 recommended inserting a wax rod into the core of the investment when investing the pattern. In this way, gasses could leave the mold cavity in a short time and be free of voids and porosities. Strickland and Sturdevant9 say that "back pressure" porosity will be eliminated if a mold temperature of over 1,100 F is used. They have also found that the type of investment can be important in the elimination of "back pressure" porosity. Strickland and Sturdevant9 contend that varying the temperature of the casting alloy has no effect on "back pressure" porosity. Sprue size, oven burnout temperature, casting temperature of the gold, presence of a vent or heat sink, and type of investment are among the variables that confront the operator in casting. Small full crowns such as those on premolars are probably the castings most often destroyed by voids and porosities. By making over 200 castings using