The buzz around 3D printing has fueled several stocks to fantastic gains, including 3D Systems (DDD) and Stratasys (SSYS). A bit below the radar you'll find a related company, Proto Labs (PRLB), a Minnesota-based small cap that manufactures custom parts as part of a prototyping or short-run product-development phase. Proto Labs bills itself as the "world's fastest source for custom CNC (computer numerical control) machined parts and injection