During the early stages of solidification of a casting, a thin skin begins to form at the relatively cool mold walls and as time passes, the thickness of the skin increases (as shown in figure 2). For flat mold walls, thickness is proportional to the square root of the solidification time. Thus, doubling the solidification time will make the skin 21/2 = 1.414 times or 41% thicker.
The solidification time of a casting is a function of the volume of a casting and its surface area (Chvorinov’s rule). Solidification time of a casting is given by the formula:
Where C is the constant that reflects (a) the mold material, (b) the metal properties (including latent heat), and (c) the temperature. The parameter ‘n’ usually takes the value 2. Sometimes ‘n’ is taken values between 1.5 to 2. Thus a large solid sphere will solidify and cool to ambient temperature at a much slower rate than will a smaller solid sphere. The reason for this is that the volume of a sphere is proportional to the cube of its diameter and the surface area is proportional to the square of its diameter. Similarly, it can be shown that molten metal is a cube-shaped mold will solidify faster than in a spherical mold of the same volume.
The effects of mold geometry and elapsed time on skin thickness and shape are shown in figure 2. As illustrated, the unsolidified molten metal has been poured from the mold at different time intervals ranging from 5 to 300 seconds.
The skin thickness increases with elapsed time and the skin is thinner at internal angles (location A in the figure 2) than at external angles (location B in the figure 2). The latter condition is caused by slower cooling time at internal angles than at external angles.