Plastic Caps, Plugs, Grips | Custom Design Molding | Harman Corporation, Harman Corporation is a U.S. manufacturer of plastic caps, grips, plugs and custom molding, located in the same building in Rochester, Michigan since 1961. With over 60 years of expertise and two expansions to the building, we have become the highest quality manufacturer of dip molded caps, grips and plugs in the industry.