Chris Lau, Kapitall: Investors have been bullish on 3D printing stocks for a while, so are they starting to become overvalued? Bears are losing heavily after betting against the bullishness in 3D printing companies. 3D Systems (DDD) is up over 110% in 2013 despite a hefty short float of 20%. Read more on 3D printing from Kapitall: Could HP Make Trouble for Other 3D Printing Stocks? Not to be outdone, ExOne (XONE) has a short float of 48% but is also up 110%. Are stretched valuations enough to justify selling 3D printing stocks, or even avoiding them? As with any potential bubble, markets may remain irrational for longer than many expect. Investor interest high Strong investor interest in 3D printing is so far supporting the bullish view for this sector. Voxeljet (VJET) completed its IPO in October and raised $84 million. The firm is unique in that its 3D molds may be produced using sand. This means the firm could service castings for customers. 3D printing reach is growing The reach for 3D printing does not stop at the consumer or commercial level. At the London 3D Printshow, Thor’s hammer was produced using a voxeljet 3D printer. But not everyone is convinced that the stretched valuation in 3D printing companies will hold. Whitney Tilson, a hedge fund manager, argued that betting against 3D Systems makes sense. Market capitalization comparison 3D Systems is the largest by market cap at $7.6 billion. Stratasys (SSYS) and ExOne have a forward P/E of over 70, but ExOne is still valued at under $1B. Voxeljet, whose shares are highly volatile, is one of the smallest 3D printing plays: Click on the interactive charts below to view market capitalization over time.Conclusion Investing in 3D printing stocks can still be considered high risk, especially due to valuation. Bets against the sector could pay off, but momentum remains positive. Unless revenue growth disappoints next quarter, there are few catalysts that would support a pullback.