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We’ve recently begun tracking short interest in the publicly traded companies that either make 3D printing systems or provide design and other services using the machines. Here are short interest totals as of April 30 for 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 fell 2.9% to 26.54 million shares. Some 31% of the company’s stock is short.

Short interest in Proto Labs rose 8.4% to 1.55 million shares, or 8.2% of the company’s float.

Stratasys saw short interest fall by 7.7% in the last two weeks of April to 4.09 million shares, or about 15% of the company’s float.

Short interest in ExOne rose by 7.3% to 2.22 million shares. About 32.8% of the company’s shares are now held short.

Short interest in half these firms rose, while it fell in the other half. The rise in short interest in Proto Labs is likely the result of a rather tepid earnings report that came out during the final two weeks. The rise in short interest at ExOne could be a bet against the company’s first-quarter earnings results, due out next week.

Both 3D Systems and Stratasys saw declines in short interest in the latter two weeks of April. Even a secondary offering could not slow down 3D Systems’ 25% share price gain since April 15. Stratasys reports first-quarter results next Monday and is expected to post earnings per share of $0.38 on revenues of $98.07 million.