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We have tracked the key short interest changes as of June 14 in the following 3D printer and services companies: 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 7.3% to 25.14 million shares. Some 26.8% of the company’s stock is short.

Short interest in Proto Labs rose 6.4% to 2.16 million shares, or 11.3% of the company’s float.

Stratasys saw short interest rise by 2.6% in the first two weeks of June to 3.27 million shares, or about 11.9% of the company’s float.

Short interest in ExOne rose by 9% to 2.4 million shares. About 35.4% of the company’s shares are now held short.

All four of these stocks saw a rise in short interest, with ExOne rising the most. The big news in the sector recently was the purchase of MakerBot, a move that gives Stratasys a grip on the first 3D printing device aimed at a mass market.

Both ExOne and Proto Labs have risen more or less steadily since hitting a low in the first week of June. Short sellers have to get the timing right to profit on either of these stocks. And it looks like the short sellers might already have made their money in the final two weeks of the month, as all four stocks are climbing again.