Ratios of measured rim and shell expansion timescales (kinematical ages), i.e. and , as listed in Table 2 for objects where both values are available, vs. the effective temperatures of the respective central stars. Values from [N ii] data, where available, are offset by +500 K for more clarity. The stellar temperatures are those adopted in Table 8. IC 418 is the only optically thick object of all the shown PNe, and the shell’s shock virtually coincides with the ionisation front. For comparison, the corresponding rim and shell timescales predicted by three hydrodynamical simulations with central stars of 0.595, 0.605, and 0.625 M_⊙ already used in previous works (e.g. Schönberner et al. 2014) are also shown (cf. legend). The kinematical age ratio maxima between 50 000 and 60 000 K are typical for the transition from the optically thick to the optically thin stage, i.e. at this moment the radial position of the shell’s shock just coincides with the ionisation front. The peaks at 80 000 K (0.625 M_⊙) and 95 000 K (0.605 M_⊙), respectively, mark the passage of the He ii ionisation front through the rim-shell interface.