Industry watchdog Wohlers associates believes the 3-D printing industry will grow from a $2.2 billion industry in 2012 to eventually become a $10.8 billion industry by the end of 2021. While this certainly sets the bar high for 3-D printing stocks, the threat of a recession or period of economic uncertainty would easily derail this expectation and investors would likely face losses.

Pure-play 3-D printing companies like 3D Systems (NYSE:DDD), Stratasys (NASDAQ:SSYS), voxeljet (NYSE:VJET), and ExOne (NASDAQ:XONE) all operate with very transactional business models, meaning they only receive business when their customers have business themselves. During periods of economic uncertainly, it's also conceivable that its customers would cut budgets related to costly 3-D printing projects.

In the following video, 3-D printing analyst Steve Heller sat down with the head of Motley Fool's industrials sector, Blake Bos, to discuss why 3-D printing stocks are not recession proof. (The relevant segment can be found between 10:15 and 11:47.)