Bob and Donna,
thanks for all your input!
It is not a problem to make the parting line exactly at half of the
ball because they do come in 2 halves.

I also thought it is because of the porosity of the polystyrene which
is tini balls melted together and I
understand what you mean with undercut.
That is probably not a good surface finish to start with..

I have asked my pottery teacher in Germany who has made a lot of these
balls and he said clay slip would be also a good medium for
separation, so I might try that..
Someone else said I should put gladwrap over the balls before I put
the plaster on. That would make the surface smooth but would give me
lines where the gladwrap is "folded".
I have also thought about painting the ball with some paint or even
fluid latex which I have here.

I guess I just have to try different things and see which works best.

Thanks again hand happy pottering and/or mould making!
Cheers from Gabi