Hi,

I reduced to a simple case (see attached Hip), what should I do to avoid interpenetration while using the spring SOP on thin box for example. I tried few things but it still doesn't work.
Any help will be appreciated.

Well found a way around. I am not too happy… but that's result that counts! Isn't it?

Does it have to be a mesh?
What about converting to polygons after the spring?

You can remove backfaces using delete opertator with ‘normal’ enabled and ‘backface from’ camera