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Short interest during the two-week period ending September 30 fell on half of the four 3D printing stocks we follow. Buying interest in the 3D printing stocks lifted share prices for all of the players during the short interest period. But short sellers don’t see that situation as sustainable for two of the companies.

Short interest in 3D Systems Corp. (NYSE: DDD) rose by 5.4% to 27.75 million shares. Some 26.2% of the company’s float was short. Days to cover rose from nine to 10. In the short interest period, the share price rose nearly 15.5%. The stock’s 52-week range is $6.00 to $19.76, and shares closed at $16.69 on Tuesday, down about 5.8% on the day.

Stratasys Ltd. (NASDAQ: SSYS) saw short interest drop by 14.4% in the last two weeks of September to about 5.96 million shares, or 12.3% of the company’s float. Days to cover fell from seven to six. The share price rose nearly 12% in the period, and the stock closed at $22.76 on Tuesday, down more than 4% on the day. The 52-week range is $14.48 to $31.60.

Short interest in The ExOne Co. (NASDAQ: XONE) rose by 6.8% to 1.87 million shares. About 19.9% of the company’s shares were short. ExOne’s share price rose by about 13% in the two-week period. Its 52-week range is $6.50 to $16.15, and shares closed at $14.30 on Tuesday, down nearly 4% for the day. Days to cover slipped from 14 to 13.

Short interest in Voxeljet A.G. (NYSE: VJET) decreased by 4.3% to 578,954, with days to cover dipping from eight to six. The share price rose by about 9.6% in the two weeks to September 30. The American depositary shares closed at $4.32 on Tuesday, down about 3.6% for the day, in a 52-week range of $3.50 to $6.99.