Abstract: In one example in accordance with the present disclosure, an additive manufacturing system is described. The additive manufacturing system includes a build material deposition device to form a plastically deformable three-dimensional (3D) object by depositing layers of a thermoplastic build material to form a body of the plastically deformable 3D object. The additive manufacturing system also includes a heat channel forming device to form heat channels within the plastically deformable 3D object which heat channels, responsive to an applied stimulus, are to soften adjacent regions of the body. The additive manufacturing system also includes a fusing system to selectively harden layers of thermoplastic build material to form the plastically deformable 3D object.