ExOne (NASDAQ: XONE) shares dropped after the company missed second-quarter sales expectations and said its 2014 gross margin would be narrower than expected.

The 3D printing machine company's second-quarter gross margin narrowed to 22.3 percent, from 45 percent a year earlier. The margin was hurt by product development costs and "under absorption" of costs associated with its expanded production service center network.

The margin was also hurt by an unfavorable mix of machine sales versus non-machine revenue.

Machine sales totaled 53 percent of second quarter revenue, compared with 63 percent a year earlier. In the recent period ExOne sold six 3D printing machines.

The company said "quarter-to-quarter fluctuations" in machine sales "are necessarily indicative of larger trends."

"As is typical in the first half of the year, we did not fully absorb our infrastructure costs as machine sales are weighted to the second half," Chief Executive David Burns said in a statement.

The company said 2014 gross margin would be between 32 percent and 36 percent "to adjust for first half actual results and second half forecast."

The prediction excludes $1.5 million to $2.5 million associated with facility expansions.

Revenue for 2014 will be between $55 million and $60 million, the company said. Analysts expect revenue of $54.9 million.

Net loss for the recent period widened to $4.7 million, or $0.32 per share. Revenue grew to $11.2 million from $9.2 million.

Wall Street expected a loss of $0.14 per share on revenue of $12.1 million.

In after-hours trading, ExOne was recently down more than nine percent.

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