This type of casting uses a long hollow cylindrical tube lined with a suitable ceramic. This tube is so mounted so that it may be spun about its longitudinal axis usually in a horizontal position. In preparing lecture material on this subject, statements as to the pressure inside the casting while it is spinning have been found. Since the temperatures inside the casting are those of molten metal, I have wondered how these pressures have been determined. Knowledge of these pressures is necessary for a better understanding of the events that take place within the casting as it spins and cools.
Directional solidification progresses inward from the mold inner surface.
The high radial pressure gradient insures that full feeding of liquid metal is available to offset the solidification shrinkage as the metal cools resulting in very sound metal. This feeding of shrinkage is obtained at pressures which are many times greater than those available by conventional sand casting. To obtain similar pressures in a sand casting would require a sprue height which is impossible to obtain. Gases liberated during cooling along with dross and other low density materials are forced to the inner surface insuring very sound metal within the interior of the casting wall. These unwanted elements can be machined off during the boring of the inner surface.