Chvorinov's rule

According to the rule, a casting with a big surface area and small volume will cool more quickly than a casting with a small surface area and a large volume under otherwise comparable conditions. The relationship can be written as:[3]

${\displaystyle t=B\left({\frac {V}{A}}\right)^{n}}$

where t is the solidification time, V is the volume of the casting, A is the surface area of the casting that contacts the mold, n is a constant, and B is the mold constant.

The ratio of the casting's volume to its surface area is the modulus M:

${\displaystyle M=\left({\frac {V}{A}}\right)}$

The mold constant B depends on the properties of the metal, such as density, heat capacity, heat of fusion and superheat, and the mold, such as initial temperature, density, thermal conductivity, heat capacity and wall thickness.

The S.I. units of the mold constant B are s/m².[4] According to Askeland, the constant n is usually 2, however Degarmo claims it is between 1.5 and 2.[3][5] The mold constant of Chvorinov's rule, B, can be calculated using the following formula:

${\displaystyle B=\left[{\frac {\rho _{\mathrm {m} }L}{\left(T_{\mathrm {m} }-T_{0}\right)}}\right]^{2}\left[{\frac {\pi }{4k\rho c}}\right]\left[1+\left({\frac {c_{\mathrm {m} }\Delta T_{\mathrm {s} }}{L}}\right)^{2}\right],}$

where

T_m = melting or freezing temperature of the liquid (in kelvins),

T₀ = initial temperature of the mold (in kelvins),

ΔT_s = T_pour − T_m = superheat (in kelvins),

L = latent heat of fusion (in [J·kg⁻¹]),

k = thermal conductivity of the mold (in [W·m⁻¹·K⁻¹)]),

ρ = density of the mold (in [kg·m⁻³]),

c = specific heat of the mold (in [J·kg⁻¹·K⁻¹]),

ρ_m = density of the metal (in [kg·m⁻³]),

c_m = specific heat of the metal (in [J·kg⁻¹·K⁻¹]).

It is most useful in determining if a riser will solidify before the casting, because if the riser solidifies first then defects like shrinkage or porosity can form.[5][6]