Methods, systems, and apparatus, including medium-encoded computer program products, for generating internal structures usable in additive manufacturing include, in at least one aspect, obtaining a three dimensional (3D) model of a 3D object to be manufactured by a 3D extrusion printer; defining an infill structure for the 3D model, wherein defining the infill structure comprises defining an internal hull separated from an external hull by a separation distance, wherein the external hull corresponds to the 3D model, and defining walls between the external hull and the internal hull to form cells between the external hull and the internal hull; and providing the 3D model and the infill structure for the 3D model to generate tool path data for the 3D extrusion printer to build the 3D object in accordance with the 3D model and the infill structure for the 3D model.