Hybrid Injection–Volumetric Additive Manufacturing for Rapid Fabrication of Low-Viscosity Multi-Material and Composite Structures

In this study, a hybrid injection–volumetric additive manufacturing (HIVAM) approach is introduced to overcome key material limitations of volumetric additive manufacturing (VAM) and enable the ultrafast fabrication of composite structures and multi material parts. VAM has recently emerged as a rapid vat photopolymerization technique that addresses several key limitations of conventional additive manufacturing methods, including low printing speed, poor surface quality, the need for support structures, and anisotropic mechanical properties. Despite these advantages, VAM is currently restricted to highly transparent resins, and the use of opaque formulations or resin systems containing light-scattering fillers, as well as low-viscosity materials that are essential for composite and multi-material fabrication, remains extremely challenging. The proposed HIVAM approach integrates directional volumetric curing with synchronized in situ injection of a secondary material, enabling the fabrication of hollow, multi-material, and composite structures. Different injection algorithms and processing conditions are systematically investigated to identify stable fabrication windows and ensure dimensional integrity. The versatility of the method is demonstrated by combining materials with distinct physical states and matrices, including opaque resins, graphene oxide-reinforced resin, and injected shear-thickening fluids (STF). As proof of concept, composite parts containing up to 1.5 wt% graphene oxide and liquid-filled architectures exhibiting strain-rate-dependent mechanical behavior are successfully fabricated. The results show that HIVAM preserves the ultrafast nature of volumetric additive manufacturing while significantly expanding its material palette and architectural complexity. This work establishes a general platform for rapid multi-material and composite fabrication beyond the transparency and viscosity constraints of conventional volumetric processes.