Spatially controlling the mechanical properties of 3D printed objects by dual-wavelength vat photopolymerization

To date, the 3D printing of polymers with heterogeneous and locally controlled material properties is still a challenging area in additive manufacturing. In terms of vat photopolymerization 3D printing, the fabrication of multi-material objects typically relies on an automatic material exchange of different resin vats. However, along with the high complexity of the printing equipment, this technique suffers from a low build speed and often yields 3D printed objects with week interlayer adhesion across the various material interfaces. Herein, we use chemo-selective wavelengths to fabricate objects with multi-material properties by dual-wavelength vat photopolymerization 3D printing employing a single vat. The photopolymers’ stiffness and flexibility are conveniently controlled by two photoreactions working at two different wavelengths. In particular, a dual photocurable resin is applied containing multi-functional acrylates, which are cured by a radical induced chain growth reaction at 405 nm, and bi-functional epoxy monomers, which additionally undergo cationic curing upon UV exposure (365 nm). FT-IR experiments confirm the wavelength selective network formation whilst dynamic mechanical analysis and tensile tests give evidence of the distinctive difference of the related mechanical properties. By being able to produce soft (ε = 24%, σ = 1.0 MPa) and stiff (ε = 4%, σ = 39.1 MPa) networks with a single resin vat, we demonstrate the efficient fabrication of 3D structures with locally controlled mechanical properties using a dual-wavelength 3D printer operating at 405 and 365 nm. In contrast to previous work in this field, we were able to significantly expand the range of mechanical properties by appropriate selection of the acrylic components and to drastically accelerate the build speed by changing the cationic photoinitiator and using a customized printer with high intensity LED sources.