Simultaneously improving fabrication accuracy and interfacial bonding strength of multi-material projection stereolithography by multi-step exposure

Multi-material additive manufacturing (MMAM) takes full advantage of the ability to arbitrarily place materials of additive manufacturing technology, enabling immense design freedom and functional print capabilities. Among MMAM technologies, projection stereolithography (PSL) exhibits a great balance of high resolution and fast printing speed. However, fabrication accuracy of multi-material PSL is hindered by large overcure used to strengthen interfacial bonding weakened by chemical affinity and material-exchange process. We present a novel multi-step exposure method for multi-material PSL process to overcome this shortcoming. Firstly, the whole layer is moderately exposed producing overcure of single-material PSL level to generate geometries. Then weakened interfaces are strengthened individually with additional steps of exposure. The multi-step exposure is integrated into the already efficient materials printing order of multi-material PSL process. Curing depth and overcure of photocurable resins are modeled and characterized. Exposure required to achieve sufficient interfacial bonding of single-material interfaces built through material-exchange process and multi-material interfaces with altering materials printing order is determined with tensile tests. Microfluidic channels are used to compare fabrication accuracy of traditional single-step exposure and our multi-step exposure method. This method can be widely applied in multi-material PSL to improve fabrication accuracy in a variety of applications including microfluidic devices. [Graphical abstract]