We have studied the dynamical decay of the autocorrelation function of the 3D Ising model for different sizes L = 20-52 of spin cluster-cubes. The behaviour of the longest, ergodic relaxation time, τ e , of a finite domain below the phase transition temperature T c was mostly considered for two types of phase transition dynamics. A study of the scaling properties of τ e demonstrates a negligible difference between the types of dynamics used, but a considerable difference for different boundary conditions. In contrast to the known result for periodic boundary conditions (τ e ~ L z exp [const(Lє