In recent years, carbon fibre reinforced components have been introduced into the automotive industry in a mass production scale. A promising approach for reducing production steps while increasing the complexity of carbon fibre reinforced car body parts is manufacturing sandwich components using wet moulding. Equipping these wet moulding parts with high performance sandwich cores fabricated by milling, as implemented in the aerospace industry, does not comply with automotive requirements in terms of cost-efficient high-volume production. Recent investigations show that is possible to use near-net-shape rigid foam cores made from polyethylene terephthalate using thermoforming as an alternative core. Expanded polyethylene terephthalate is utilized for cladding or insulating in the marine industry or for wind energy applications, where foam parts are manufactured by simple bending after a heat treatment. In order to fabricate near-net-shape sandwich cores, an advanced thermoforming procedure for expanded polyethylene terephthalate needs to be developed. This contribution aims at describing the impact of the thermomechanical behaviour of the foam material and the resulting steps of the advanced procedure. Initial trials suggest that the occurrence of two major effects during thermoforming and subsequential wet moulding can result in faulty cores or sandwich parts: Firstly, the gas pressure inside the foam cells, which tend to collapse under forming but do not fracture and, secondly, the entropy elastic behaviour of polyethylene terephthalate above the glass transition temperature. In combination with external forces, both can result in delamination in the sandwich part. The development of an advanced thermoforming procedure aims at circumventing the negative impact of both effects. After the initial forming, smaller parts of the rigid foam core are reheated shortly above the melting point in order to locally destroy the foam cell structure. This results in rigid foam cores that are suitable for the wet moulding of sandwich parts without causing delamination or other failures.