Material extrusion (ME) can be used to manufacture a range of three-dimensional shapes by stacking extruded polymer layers with a heated nozzle. While the process is suitable for customized production in small quantities, the use of ME-printed features is often limited with regard to mechanical purposes due to the intrinsic anisotropic strength shortcoming, as parts can easily failure against a load in the transverse direction. In addition, the production speed for multiple parts is another major drawback preventing real competition with other mass manufacturing technologies. In this paper, we introduce an additive manufacturing strategy of shell pre-printing and core post-casting (referring as shell-core printing) that effectively enhances the isotropic strength and the production speed. Shell-core printing involves 3D printing a thin shell of the three-dimensional shape using the conventional ME method, followed by the injection and curing of a reinforcement resin inside the core. We show that the transverse strength of the layered shell can be reinforced by the core and that the reinforcement can be more effective when the shell-core interface roughness is effectively removed. Using an additional surface-polishing step between the pre-printing and the post-casting, the degree of isotropy with regard to the strength can be enhanced significantly from 0.4 to nearly 1, while the overall production time can also be reduced by half that in the conventional ME process. We also investigate the tensile behavior of the shell-core printed specimen and experimentally verify that the characteristics in general follow the classical theory of the rule of mixtures. In addition, we find that many different fracture modes exist in shell-core printed parts depending on the adhesive strength at the shell-core interface. We provide a theoretical criterion for the interfacial bonding energy which determines the different fracture behaviors and experimentally validate this in a mode II fracture test.