Expansion properties of the model nebulae of the six α = 3 sequences with different metallicities vs. the post-AGB age. The five black dots along the abscissas refer to the snapshots depicted in Figs. 5−9. The initial velocities of the models are 10 km s^-1 (cf. Fig. 2). Left: propagation rates of the outer shocks, Ṙ_out, which define the nebular sizes. The velocity jump of the 3   Z_GD model at t ≃ 13000 yr marks the moment when the faster expanding rim starts overtaking the outer shock, and afterwards. Right: the gas velocities immediately behind the outer shock. Again, the velocity jump of the 3   Z_GD model, now already at t ≃ 12000 yr, is due to the disappearing shell.