A method and apparatus is disclosed for joining of composites with thermoplastic surfaces. The process includes the location of the said components within a load bearing frame, and bringing the thermoplastic surfaces to be joined into contact. Pressure application devices, in the form of fluid filled bladders, are also located within the frame, and are located such that pressure is applied evenly to said thermoplastic surfaces. Where components have a high level of curve or twist, form blocks that have the approximate shape of the component surface are located between the frame and bladders. Pressure and heat are applied to the joint area, and heat removed while the joint area is maintained under some pressure. A feature of the present invention is the option of using opposing pressure application devices, reducing the need for substantial tooling in order to maintain component location accuracy. A distinct advantage of this invention is the flexibility to join curved or twisted components, or complete the welding of closed assemblies such as wing boxes.