This method is used to mould integrally stiffened structures with complex forms. The process involves wrapping prepreg layers over blocks of rubber or foam, which are then restrained in a metal cavity and heat applied. As the temperature increases, differential thermal expansion occurs between the metal and the rubber, i.e. the rubber expands more than the metal, creating extremely high pressure. The combination of pressure and heat consolidates the lay-up.

Components with complex shapes can be moulded in a single cure cycle, thus reducing the number of joints and parts. This can lead to a significant decrease in weight and reduced production costs.

A variant of this process makes use of a solid mandrel fabricated from a material known to have a very high coefficient of thermal expansion, e.g. silicone rubber. This mandrel is wrapped in prepreg and placed in a simple, rigid tool. The assembly is then heated and, during the curing process, the mandrel expands, generating consolidation pressure. When the assembly has cooled, the mandrel can be extracted to leave a hollow component.
 
This method requires minimal capital equipment, and can be performed with simple, low cost tooling.