Topology optimization is broadly recognized as a design approach to generate high-performance conceptual designs suitable for freeform fabrication, e.g., additive manufacturing. When other fabrication methods are considered, topology optimization must integrate manufacturing constraints. The integration of constraints for extrusion and casting has been addressed in the past by a few researcher groups. In this work, extrusion and casting constraints are revisited and extended to include plastic injection. The proposed method relies on the use of intersection planes and the definition of a parting line within the planes. The resulting topologies can be injected in a two-plate mold without the use of inserts. The implementation and results of the proposed approach are demonstrated in classic three-dimensional problems that include a cantilevered beam with different load conditions.