Industrial firms hired 31,000 people in November, which is the second biggest increase in four years. Only a 44,000 person gain in August was bigger.

Over the past year manufacturers have added almost 200,000 new jobs. That’s the biggest 12-month increase since 2015.

What’s more, manufacturers have now added just over 1 million new jobs since industry employment fell to a post-World War II low of 11.45 million in early 2010. These companies now employ 12.5 million Americans.

Manufacturers have managed to add workers despite a growing shortage of skilled laborers. The unemployment rate stands at a nearly 17-year low of 4.1% and there simply aren’t that many trained welders, iron workers, mechanical engineers and the like available for hire.

GDP has topped 3% two quarters in a row and the need to rebuild low inventories are stoking demand for manufactured goods. Surging U.S. oil and gas production in particular has been a boon for manufacturers.

The strongest global economy in years is also lending a hand. U.S. exports have soared over the past year.

The biggest problem for manufacturers is finding more workers. Some companies have resorted to training new employees, sending them to vocational school or using other means to fill in the gaps. Others are buying more robots or using automation to help boost production.

Unless the labor shortage is addressed, however, manufacturers could hit a wall.

Read more at marketwatch.com.