Techniques and systems for computer aided design of physical structures using an object splitting design process that optimize manufacturing efficiency are described. A described techniques includes obtaining an input model of an initial object to be manufactured, the input model providing a 3D topology and shape for the object; determining, based on the 3D topology and shape, a 3D parting surface to split the initial object into separate objects, the parting surface intersects an inflection zone that is situated between up and down skin regions of the 3D topology and shape; and providing first and second 3D models of the separate objects in accordance with the 3D topology and shape and the parting surface, for use in manufacturing separate physical structures corresponding to the separate objects using one or more computer-controlled manufacturing systems, where after respective manufacturing, the physical structures are configured to be assembled to form a combined structure.