A method for manufacturing a sandwich structure, which sandwich structure comprises at least one foam layer (12; 12a, 12b) of a first thermoplastic and two fiber-reinforced cover layers (20, 22) of a second thermoplastic, the method comprising the steps of: a) providing a starting structure comprising at least one layer of a first thermoplastic having a melting temperature or melting range, and two fiber-reinforced cover layers (20, 22) of a second thermoplastic; wherein the at least one layer of the first thermoplastic comprises a chemical blowing agent having a decomposition temperature above the melting temperature or melting range of the first thermoplastic, and has been manufactured by extrusion or calendaring; b) contact heating the starting structure to a temperature above the decomposition temperature of the chemical blowing agent so that decomposition of the chemical blowing agent occurs, thereby obtaining an intermediate structure wherein the decomposed chemical blowing agent is present in the at least one layer of the first thermoplastic; c) subsequent to the decomposition of the chemical blowing agent cooling of the intermediate structure thus obtained to a foaming temperature above the melting temperature or range of the first thermoplastic; wherein said contact heating step b) and cooling step c) are carried out under pressure and wherein the starting structure and intermediate structure are in contact with the press tools thereby preventing foaming of the at least one layer of the first thermoplastic d) directly following step c) at the foaming temperature foaming of the at least one layer of thermoplastic comprising the decomposed chemical blowing agent of the intermediate structure, wherein the volume of the intermediate structure is made to increase to a final volume and then kept constant thereby obtaining the sandwich structure, which structure comprises the at least one foam layer (12; 12a, 12b) of the first thermoplastic and the two fiber-reinforced cover layers (20, 22) of the second thermoplastic; and e) cooling of the sandwich structure thus obtained at a constant final volume. a) providing a starting structure comprising at least one layer of a first thermoplastic having a melting temperature or melting range, and two fiber-reinforced cover layers (20, 22) of a second thermoplastic; wherein the at least one layer of the first thermoplastic comprises a chemical blowing agent having a decomposition temperature above the melting temperature or melting range of the first thermoplastic, and has been manufactured by extrusion or calendaring; b) contact heating the starting structure to a temperature above the decomposition temperature of the chemical blowing agent so that decomposition of the chemical blowing agent occurs, thereby obtaining an intermediate structure wherein the decomposed chemical blowing agent is present in the at least one layer of the first thermoplastic; c) subsequent to the decomposition of the chemical blowing agent cooling of the intermediate structure thus obtained to a foaming temperature above the melting temperature or range of the first thermoplastic; d) directly following step c) at the foaming temperature foaming of the at least one layer of thermoplastic comprising the decomposed chemical blowing agent of the intermediate structure, wherein the volume of the intermediate structure is made to increase to a final volume and then kept constant thereby obtaining the sandwich structure, which structure comprises the at least one foam layer (12; 12a, 12b) of the first thermoplastic and the two fiber-reinforced cover layers (20, 22) of the second thermoplastic; and e) cooling of the sandwich structure thus obtained at a constant final volume.