I had a similar problem with cut3d, although I was working in wood. I worked round it by modifying the shape slightly so I had a slope just off vertical, although I was lucky that that suited the piece I was cutting. I would be interested to hear if there are any better fixes - I spent a lot of time modifying the stl model as the vertical face was curved in plan view and the original model came from a 3D scan and a very irregular mesh.