I found this tutorial on how to change specific parts of an object so you can make it stronger or more flexible than the rest of the object.

First you have to create a modifier and load it into Slic3r

This is the result of the modifier mesh.

This is the original object and it can be clearly seen what the difference is.

I think it might be useful to 3D print some objects with different densities.

Have a look at the full article HERE: