I don't have time right this moment to investigate, but generally "one object works, but disappears as soon as you add another object" means that the object is malformed.  With only one object CGAL (the fancy geometry processor) doesn't get involved, but with two it does and it doesn't like malformed objects. If you have such a malformed object, adding in even something simple like a cube will demonstrate the problem. Module structure - what is in a module and what is not - does not affect the situation.  What matters is the 3D shapes and how they are combined. Note that once you get past this it looks like you will have preview rendering problems.  Parts will be transparent.  Your object is not convex and you have not said "convexity=xxx" in your linear_extrude call.  Just set convexity to 10 or so.