A seat back assembly including a frame molded from material having low density such as expanded polypropylene. The seat back assembly further includes an adjustable head rest assembly including a guide bracket encapsulated in a frame. Encapsulating the guide bracket within the frame provides the headrest assembly with a rigid support, while simultaneously reducing the mass of the seat back assembly with respect to seat back assemblies having a metal seat frame. The headrest assembly includes a headrest body and a pair of legs extending outwardly from the headrests. The guide bracket is fully encapsulated within the frame during the molding process, and is configured so as to slidingly receive each of the legs and secure the headrest assembly to the seat back assembly.