In my experience, workable living hinges can be made through FFF/FDM 3D printing. However, they will not nearly be half as strong and durable as IM'ed hinges due to the filament structure, material and heterogeneities. The best idea to me seems to use a dual extruder and print the hinge in a flexible material, with an uneven number of layers - 3 or higher - the outer layers being aligned with the bending direction.