Insiders at Proto Labs Inc., which has more than doubled in value since it went public in February, plan to sell about $100 million of their stock to the public, according to a filing late Wednesday with the U.S. Securities and Exchange Commission.

The preliminary documents do not yet detail which shareholders will sell or how much each will sell.

The largest shareholders include Minnesota-based North Bridge Growth Equity, an investment firm that owns 4.8 million shares; 11-year CEO Brad Cleveland, 1.4 million shares; and founder Larry Lukis, 6.9 million shares.

Proto Lab shares closed at $34.76, up 6 cents on Thursday. It has a market value of about $835 million. It went public Feb. 23 at $16 per share.

The Maple Plain-based company makes custom parts for prototyping and short-run production for manufacturers around the globe. It was started in 1999 by Lukis, a longtime inventor who was seeking a better, faster way to make parts for copiers and other equipment.

The company reported income from operations of $24.7 million on revenue of $92.4 million during the first nine months of 2012.

Neal St. Anthony • 612-673-7144 •