There's been a tremendous amount of hype in recent years over 3-D printing -- the process of manufacturing by adding successive layers of material on top of each other to create a finished product -- as awareness of the futuristic technology has grown among mainstream consumers. Images of desktop 3-D printers cranking out custom-designed plastic doodads and widget-like toys and phone cases, as if by magic, have captured the public's imagination.

For investors, however, these low-quality, mass audience applications may simply be too easy for competitors to enter to provide any lasting profitability. That's why the giants of the 3-D printing industry are racing to enter more technically complex, high-value industries, and chief among them is mastering the art and science of printing with metal.

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What is this even for? There's no margin here. Photo credit: FormLabs

3D Systems (NYSE:DDD) and Stratasys (NASDAQ:SSYS) are the two largest companies in the world that are focused purely on 3-D printing and, for years now, the companies have been investing heavily in expanding the portfolio of materials for their printers. Each company has more than 100 materials, from inexpensive brittle colored plastics suitable for not much more than making an eggcup, to bio-compatible polymers that can be used to create medical implants.

As the technology to create the lowest-value end of the spectrum becomes ever more commoditized, the two companies have focused aggressively on expanding their offerings of more demanding print materials that allow their products to find applications in highly profitable fields, like medical device engineering, and rapid prototyping -- the research and development strategy of quickly producing small numbers of product designs to see how the product performs under real-world conditions. Stratasys and 3D Systems have already found a market for rapid prototyping with plastic components, but being able to print in metal opens up vast new rapid prototyping opportunities in lucrative fields like aeronautics, automotive engineering, and defense.

The big guys might have been spurred to action when ExOne (NASDAQ:XONE), a small 3-D printing company that's focused primarily on creating metal print solutions for industrial customers, made its public debut in February. Over the eight months since its initial public offering, the upstart ExOne clocked an astonishing 90% stock gain, about six times what 3D Systems and Stratasys managed. In June of this year, both companies signaled their intentions to compete more heavily for the metal market.

3D Systems moved fastest, announcing its acquisition of French metallic print specialist Phenix Systems, along with the 50 or so patents that the company owns related to printing with metal and ceramics. The deal was a relatively small one, costing 3D Systems about $20 million of the more than $250 million that 3D Systems had raised through a stock offering the month before. However, the acquisition is expected to be profitable as soon as 2014, with help from 3D Systems' impressive distribution and sales channels. The biggest long-term advantage to 3D Systems will be in incorporating Phenix's expertise in, and intellectual property about, printing with metals into its own product portfolio.

Stratasys has been moving a bit slower than 3D Systems, having just completed the largest merger in the 3-D print space in 2012 by combining with Israel-based Objet, which, at the time, was the world's third largest 3-D printer. While managing that large-scale integration, Stratasys also went out and bought MakerBot, one of the most visible consumer-facing 3-D printers. Despite these distractions, in June, Stratasys founder and chairman Scott Crump told Bloomberg that his company was looking to expand into metals through both in-house efforts and acquisitions. Last month, that desire became more of a reality, when Stratasys completed a new stock offering that netted the company nearly half a billion dollars in capital, which management has indicated will be used to fund further acquisitions.

So, who's on the chopping block? Despite Stratasys boasting about seven times the sales of its smaller competitor, ExOne looks to be a bit big for Stratasys' price range. Stratasys would be expected to pay a hefty premium on the present value of any successful company in the fast-growing 3-D printing industry and, with ExOne's market capitalization currently at nearly $700 million, Stratasys would need to raise a lot more capital to finance a takeover. Sweden-based Arcam might be a contender, with unique patents centered on metals printing, and a more manageable market capitalization of $450 million; but, after so many large acquisitions, Stratasys may wish to emulate 3D Systems, and play it small. Whatever Stratasys acquires next, you can bet it will be in metals, and you can bet it will be soon. It can't afford to leave the field to its competitors for too long.

Fool contributor Daniel Ferry owns shares of 3D Systems and Stratasys. The Motley Fool recommends 3D Systems, ExOne, and Stratasys. The Motley Fool owns shares of 3D Systems, ExOne, and Stratasys and has the following options: short January 2014 $36 calls on 3D Systems and short January 2014 $20 puts on 3D Systems. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.