A method comprising: by a 3-dimensional, 3D, core design engine of a computing system (100, 600): identifying, in a computer-aided design, CAD, model, an under-core ply of a composite part and a core footprint on the under-core ply, the under-core ply and core footprint specified for an additive 3D core to be manufactured via additive manufacturing and to be inserted into the composite part; computing a bottom core surface of the additive 3D core from the under-core ply and core footprint by setting a shape of the bottom core surface as a shape of a portion of the under-core ply as outlined by the core footprint; accessing core design parameters for the additive 3D core, the core design parameters comprising core thickness parameters (311, 312, 313) and core shape parameters (331, 332) that define the additive 3D core; and constructing an additive 3D core design in the CAD model for the additive 3D core based on the computed bottom core surface and the core design parameters; and by a 3D core printing engine of the computing system (100, 600): storing the additive 3D core design; providing the additive 3D core design to printing hardware; and manufacturing by the printing hardware, via additive manufacturing, the additive 3D core with the additive 3D core design. by a 3-dimensional, 3D, core design engine of a computing system (100, 600): identifying, in a computer-aided design, CAD, model, an under-core ply of a composite part and a core footprint on the under-core ply, the under-core ply and core footprint specified for an additive 3D core to be manufactured via additive manufacturing and to be inserted into the composite part; computing a bottom core surface of the additive 3D core from the under-core ply and core footprint by setting a shape of the bottom core surface as a shape of a portion of the under-core ply as outlined by the core footprint; accessing core design parameters for the additive 3D core, the core design parameters comprising core thickness parameters (311, 312, 313) and core shape parameters (331, 332) that define the additive 3D core; and constructing an additive 3D core design in the CAD model for the additive 3D core based on the computed bottom core surface and the core design parameters; and identifying, in a computer-aided design, CAD, model, an under-core ply of a composite part and a core footprint on the under-core ply, the under-core ply and core footprint specified for an additive 3D core to be manufactured via additive manufacturing and to be inserted into the composite part; computing a bottom core surface of the additive 3D core from the under-core ply and core footprint by setting a shape of the bottom core surface as a shape of a portion of the under-core ply as outlined by the core footprint; accessing core design parameters for the additive 3D core, the core design parameters comprising core thickness parameters (311, 312, 313) and core shape parameters (331, 332) that define the additive 3D core; and constructing an additive 3D core design in the CAD model for the additive 3D core based on the computed bottom core surface and the core design parameters; and by a 3D core printing engine of the computing system (100, 600): storing the additive 3D core design; providing the additive 3D core design to printing hardware; and manufacturing by the printing hardware, via additive manufacturing, the additive 3D core with the additive 3D core design. storing the additive 3D core design; providing the additive 3D core design to printing hardware; and manufacturing by the printing hardware, via additive manufacturing, the additive 3D core with the additive 3D core design.