An additive manufacturing method for improved core structure includes splitting a computational model into sequentially queued individual model segments based on a predetermined number of slices with a predetermined orientation, and building a part in layers applied sequentially to form a final part having a core portion supported with a custom in-fill pattern. An additive manufacturing system for providing an improved core structure supported by a custom in-fill pattern includes a controller having instructions for individual model segments based on slices of a solid model, and a 3D printer for sequentially printing individual model segments in adjacent layers. A 3D-printed part includes a first exterior surface separated by a gap from a second exterior surface, and an internal core structure within the gap for mechanically connecting the first exterior surface to the second exterior surface. The internal core structure includes a custom in-fill pattern of support members.