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We apply an axially symmetric pseudo-3D photoionization model, pyCloudy, toderive the structures of 6 bipolar nebulae and 2 suggested post-bipolars in aquest to constrain the bipolar planetary nebulae evolution. HST images andVLT/UVES spectroscopy are used for the modelling. The targets are located inthe direction of the Galactic bulge. A 3D model structure is used as input tothe photoionization code, so as to fit the HST images. Line profiles ofdifferent ions constrain the velocity field. The model and associated velocityfields allow us to derive masses, velocities, and ages.  The 3D models find much lower ionized masses than required in 1D models:ionized masses are reduced by factors of 2-7. The selected bi-lobed planetarynebulae show a narrow range of ages: the averaged radii and velocities resultin values between 1300 and 2000 yr. The lobes are fitted well with velocitieslinearly increasing with radius. These Hubble-type flows have been foundbefore, and suggest that the lobes form at a defined point in time. The lobesappear to be slightly younger than the main (host) nebulae, by ~500 yr, theyseem to form at an early phase of PN evolution, and fade after 1-2 kyr. We findthat 30-35% of bulge PNe pass through a bipolar phase.

3D pyCloudy modelling of bipolar planetary nebulae: evidence for fast fading of the lobes