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 cover layers (20, 22), the method comprising the steps of: a) providing an advancing starting structure comprising at least one layer of a first thermoplastic having a melting temperature or melting range, and two cover layers (20, 22); 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 is manufactured by extrusion or calendaring; b) heating the advancing 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 advancing 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 advancing intermediate structure thus obtained; wherein said heating step b) and cooling step c) are carried out under pressure thereby preventing foaming of the at least one layer of the first thermoplastic; d) during further advancing of the intermediate structure and at a temperature above the melting temperature or melting range of the first thermoplastic 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 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 cover layers (20, 22); and e) cooling of the advancing sandwich structure thus obtained. a) providing an advancing starting structure comprising at least one layer of a first thermoplastic having a melting temperature or melting range, and two cover layers (20, 22); 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 is manufactured by extrusion or calendaring; b) heating the advancing 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 advancing 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 advancing intermediate structure thus obtained; wherein said heating step b) and cooling step c) are carried out under pressure thereby preventing foaming of the at least one layer of the first thermoplastic; d) during further advancing of the intermediate structure and at a temperature above the melting temperature or melting range of the first thermoplastic 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 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 cover layers (20, 22); and e) cooling of the advancing sandwich structure thus obtained.