A method of forming and assembling a foldable plastic utility enclosure using molds to form sides of the enclosure, preferably using glass fiber-reinforced thermoplastic composite. The molds are constructed to form a central hole in hinge members which are formed on ends of the sides. The central hole of the hinge member is molded with two molding parts, wherein one or both of the molding parts have a solid central portion that forms the central hole. The sides are removed from the molds after curing and the sides are assembled to form the utility enclosure while the sides are still hot from the molding process. Hinge pins are inserted into the central holes of the hinge members to form hinges and to prevent the sides from warping during cooling. Assembling the sides and inserting hinge pins to form the utility enclosure is completed within approximately 10 minutes after removing the sides from the molds.