Each day at Zimmer Biomet headquarters in Warsaw, IN, machinists on one robot-assisted factory floor churn out about 3,000 metallic knee parts. They are facing pressure to crank up the pace as the population ages and demand soars.

But the artificial-bone giant is grappling with a steep downside of the nation’s low unemployment rate: It is struggling to find enough workers, despite offering some of the region’s best pay and benefits.

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