A subsidiary of shoe giant Nike that makes the plastic air pockets in shoes is expanding its St. Charles County facility in the Missouri Research Park.

Nike Air Manufacturing Innovation plans to start late next month on a 75,000 square-foot expansion of its factory at 8 Research Park Drive. The 222,575 square-foot plant employs about 640 people and complements a plant near Nike’s corporate headquarters near Portland, Ore.

Nike Air MI told St. Charles County officials, which signed off on the expansion last week, that its plant here was at full capacity after adding additional equipment and employees over the last year. In an application to the county, Nike Air said that the expansion would allow it to create additional jobs, though a number wasn’t specified. A representative from Nike could not be reached.

Construction will take about 18 months, according to St. Charles County.

“Nike Air Manufacturing Innovation is part of the strong and growing plastics sector in St. Charles County,” St. Charles County Executive Steve Ehlmann said in a statement.

Nike Air MI, a subsidiary of Nike, also makes custom plastic products for customers outside of Nike. Originally founded in 1964 as Tetra Plastics, Nike purchased the company in 1991.