We have tracked the key short interest changes as of June 28 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% to 26.9 million shares. Some 28.7% of the company’s stock is short.

Short interest in Proto Labs rose 21.1% to 2.61 million shares, or 13.7% of the company’s float.

Stratasys saw short interest rise by 5.2% in the final two weeks of June to 3.44 million shares, or about 12.5% of the company’s float.

Short interest in ExOne fell by 2.7% to 2.34 million shares. About 34.4% of the company’s shares are now held short.

Short interest in ExOne dropped after the stock started climbing off lows posted early in June. This may be due partly to an announcement by Microsoft Corp. (NASDAQ: MSFT) that it would offer a native API for 3D printer support.

Stratsys’s purchase of MakerBot, and the stock’s rising price, undoubtedly brought out the skeptics who do not see a big boost for the company as a result of the acquisition. With the shares having risen by about $15, expect some short covering.