A method and apparatus for forming a stiffened thermoplastic panel is presented. The method may comprise placing a number of forming tools within a number of concave areas of a thermoplastic preform. The method may also place the thermoplastic preform with the number of forming tools within the number of concave areas on a thermoplastic lay-up to form a workpiece. The method may also position the workpiece within a tool. The tool may have a number of die liners configured to generate heat in response to a magnetic field. The method may also consolidate the workpiece to form the stiffened thermoplastic panel. Consolidating may comprise applying a magnetic field to the number of die liners to heat the workpiece to a consolidation temperature.