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We recently began tracking short interest in the publicly traded companies that either make 3D printing systems or provide design and other services using the machines. As of April 15, short interest has risen in 3D Systems Inc. (NYSE: DDD), Stratasys Ltd. (NASDAQ: SSYS), The ExOne Co. (NASDAQ: XONE) and Proto Labs Inc. (NYSE: PRLB).

Short interest in 3D Systems rose 2.9% to 27.32 million shares. Some 32.2% of the company’s stock is short.

Short interest in Proto Labs rose 37.8% to 1.43 million shares, or 7.6% of the company’s float.

Stratasys saw short interest rise by 7% in the first two weeks of April to 4.43 million shares, or about 16.2% of the company’s float.

Short interest in ExOne rose by 23.1% to 2.07 million shares. About 32.9% of the company’s shares are now held short.

Short interest in the 3D printing companies continues to rise as the bloom fades. The big question is how quickly the technology will be adopted. Forward earnings multiples are falling back to earth, and that is reflected in the fact that both 3D Systems and ExOne now have about a third of their shares held short.

Proto Labs reported first-quarter results this morning that were above estimates for both earnings and revenues, but the beats were relatively narrow. That likely will only inflame short interest in the company’s stock.