a process is presented for fabricating composite bodies of thermoplastic material including a foamed core bonded to a compatible thermoplastic sheet . a sheet of thermoplastic material is first preheated and then thermoformed on a first half - mould . the first half - mould with the thermoformed sheet is positioned opposite a second half - mould to form a hollow chamber therebetween . a foamable thermoplastic material containing one or more liquid hydrocarbons is injected into the hollow space at a temperature sufficiently high to permit the foamable material to expand and bond to the thermoformed sheet .