We have 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. As of March 15, short interest has risen in 3D Systems Inc. (NYSE: DDD), Stratasys Ltd. (NASDAQ: SSYS), ExOne Co. (NASDAQ: XONE) and Proto Labs Inc. (NYSE: PRLB).

Short interest in 3D Systems rose 7.9% in early March to 25.95 million shares. Some 31.4% of the company’s stock is short.

Short interest in Proto Labs rose 0.6% to 1.032 million shares, or 5.7% of the company’s float.

Stratasys saw short interest rise by 20.4% in the first two weeks of March to 3.51 million shares, or about 12.9% of the company’s float.

Short interest in ExOne rose from about 1.24 million on February 28 to 1.54 million shares by the middle of March. Of 6.31 million shares floated, about 23.7% are short.

Short interest continues to rise in all these stocks, largely because many investors think they are overbought. ExOne, which came public in January, reports quarterly earnings after markets close today and is expected to post EPS of $0.08 on revenues of just $12.3 million. The company’s forward P/E ratio is north of 225. Stratasys reported results earlier this month and beat estimates, but the forecast was soft and the company’s forward P/E ratio now sits at around 30. 3D Systems has seen its shares lose more than 10% of their value since reporting earnings in late February and splitting the shares two-for-one. The company’s forward P/E is around 24.