A process and device for 3D printing parts incorporating long-fiber reinforcements in an advanced composite material is disclosed. A nozzle for a 3D printing device receives a polymer material and a reinforcing fiber through separate inlets. A passage from the reinforcing fiber inlet cleaves the passage containing the polymer material, creating an interstitial cavity into which the reinforcing fiber is introduced. The polymer material closes back on itself and encapsulates the reinforcing fiber, then drags the fiber along with the flow and exits nozzle to be deposited on a work surface or part being manufactured.