We propose to exploit recent advances in GPU and Computer Graphics to accelerate the slicing process of objects modelled via a CSG language. Our target are open source low cost FDM printers such as RepRaps. Our approach first inputs a CSG description of a scene which can be composed of both meshes and analytic primitives. During display and slicing the CSG model is converted on-the-fly into an intermediate rep- resentation enabling fast processing on the GPU. Slices can be quickly extracted, and the tool path is prepared through image erosion. The in- teractive preview of the final geometry uses the exact same code path as the slicer, providing an immediate, accurate visual feedback. We propose a complete implementation of our technique, available for download. We discuss several advantages of our approach such as modeling with multiple materials, analytic primitives and slice shaders.