Plastic scintillator detectors with 3D granularity and sub-nanosecond time resolution offer simultaneous particle tracking, identification, and calorimetry. However, achieving fine segmentation at scale remains a major challenge due to high manufacturing costs, extended production timelines, and stringent precision requirements. To overcome these barriers, the 3DET R&D collaboration has developed a novel additive manufacturing technique that enables the monolithic fabrication of finely segmented 3D scintillators, thereby eliminating the need for complex assembly and additional processing. A prototype was fabricated using Fused Deposition Modeling (FDM), consisting of a 5 × 5 × 5 matrix of optically isolated scintillating voxels. The design integrates transparent polystyrene, 3D-printed reflective structures, and orthogonal 1 mm holes to accommodate wavelength-shifting fibers. The detector’s performance was evaluated in a test beam at CERN’s Proton Synchrotron, demonstrating high light yield, minimal optical crosstalk, and efficient particle detection. To enhance light confinement, a new white reflective filament was developed, showing improved reflectivity and transmittance We will present recent advancements and experimental results from prototype characterization. This work establishes a scalable, cost-effective, and time-efficient approach for producing next-generation scintillator detectors with arbitrary geometries, enabling compact, modular, and high-performance particle detection systems.