Abstract: A system and method for forming a composite structure involves providing at least two polymeric sheets as outer layers such that a cavity is formed therebetween, adhesively bonding fibrous layers to the polymeric layers to hold the fibrous layers in place during processing, and injecting into the cavity a polymeric core material capable of exhibiting a foamed character and a resinous character. The cavity is evacuated during filling with the core material, and a blowing agent of the core material is activated upon exposure to the evacuated cavity. The core material contacting the fibrous layers is reconverted to its resinous character and forms a relative dense, resinous interface between the core and fibrous layers. Upon filling the region of the cavity between the fibrous layers, a catalytic reaction is initiated in the core.