A Minnesota company that's developed a next-generation, rapid-turnaround system to deliver plastic molds and parts for manufacturers has scored one of the biggest equity investments in a private Minnesota company in recent years.

North Bridge Growth Equity of Boston and Silicon Valley, Calif., has invested $52 million for an unspecified minority stake in Maple Plain-based Proto Labs Inc., the company will announce Tuesday. Proto Labs is the global leader in rapid injection molding and computer-controlled rapid prototyping, according to the company and trade press.

The latest investment is more than three times larger than the approximately $15 million in existing equity raised from executives and Minneapolis-based Private Capital Management, an early-stage investor.

The company expects to post revenue of about $50 million and employ nearly 300 people in North America, Europe and Japan by the end of this year.

Brad Cleveland, Proto Labs chief executive since 2001, said the funds will be used for technology development, expansion and buying back some stock from original shareholders. However, Cleveland said existing shareholders will continue to own majority control of the company.

Cleveland said the company has been profitable since 2002 and has no imminent plans to sell shares to the public.

Proto Labs, through its Protomold and First Cut Prototype divisions, has received national attention in industry press for cutting the lead time for providing low-volume production of injection-molded plastic parts and computer numerically controlled (CNC) machined-prototype parts through the use of its proprietary software.

"On the outside, we look like a glorified, quick-turn job shop," Cleveland said in an interview. "Once you get inside, we are a very high-performance software development company. We run it on computing systems that we built ourselves. We've automated most of the manual labor out of the process. Most of the Protomold side, the tool-making side in our industry by other companies, has gone to China. We use our software and parallel-processing to automate a lot of that design and manual programming to cut out those molds. We do it here in Maple Plain, 20 miles west of downtown Minneapolis, and in Telford, England."

A customer -- Xerox for example, or a medical products company -- might order 50 parts for a new machine by electronically sending a computer-aided design file to the Proto Labs "First Cut" website, which immediately provides a quote on the job.

In as little as a day after an initial order, First Cut machines the parts and provides one set of all the plastic parts ordered. Xerox engineers then assemble the prototype copier to make sure it fits together. Then, they'll go back to Proto Labs for 50 to 100 copies of each part to assemble test machines.

"Assuming he designed the product right, that Xerox engineer will get the molds and parts for each of the plastic products he ordered within five or six days," Cleveland said. "Nobody else can do 50 or more molds and deliver all those parts within a week or two.

"We are proven, profitable for years and very accepted in North America and Europe. We will get Japan running next year and we're working with thousands of customers, none of whom represents more than 2 or 3 percent of revenue."

North Bridge is a seasoned investor in the manufacturing-software industry, including a founding investor in SolidWorks, a leading 3D computer-aided design software company.

"Design engineers and product development teams worldwide realize a highly compelling value from Proto Labs' technology and services," said Doug Kingsley, manager general partner of North Bridge.

A North Bridge representative has joined Cleveland's board.

"We want to be the fastest, easiest and coolest way for design engineers to get prototype or low-volume production parts," said Cleveland, 48, a veteran technology executive.

Proto Labs manufacturing employees make up to $20 per hour plus bonuses and company-paid health insurance. The company has been praised by state officials for taking high school graduates and training them for what can be lucrative technical careers.

The company was founded as Protomold in 1999 by Chief Technical Officer Larry Lukis, 60. He had grown frustrated with the time and expense involved with securing prototype molds for the plastic components used in printers that he once designed for the former LaserMaster Technologies.

Neal St. Anthony • 612-673-7144 nstanthony@startribune.com