Hello
For a 3d print I need to add thickness on this model but it crosses itself.
Please tell me that there is a way to fix this ? 
ok so no way to avoid self collision !?! it seems to be a smart and obvious option though 
Hello @MrCedric ,
Please keep in mind that in order to work with Dynbamesh, a mesh must be a closed (watertight) volume with a degree of thickness. If a mesh is too thin or overly 3d in an area, it will begin to break apart and create problematic geometry. At a glance, this mesh does not appear optimal for Dynamesh, and is likely to produce problems.
It looks like what is happening here is that your mesh is fairly thin, and opposing surfaces are poking through to the other side. It may be that simply Dynameshing this mesh will remove those overlapping areas, but I’m not optimistic.
However, this mesh is complicated and not an optimal case for Dynamesh, and it may very well produce problem geometry in those areas. It may be better to clean up and simplify that mesh prior to using Dynamesh on it. Make sure that the mesh has a minimum degree of thickness in every area, and eliminate the areas where one surface is clipping through another.
In fact I found a solution in Blender, a simple check option 