Effect of Printing Parameters on Cross-Linked Polymer Networks—An Investigation into Additive Manufacturing

The near limitless possibilities for processing complex geometries that additive manufacturing (AM) allows for has generated enormous interest in recent years. Initially, three-dimensional (3D) parts were printed via fused layer deposition manufacturing (FDM), but due to poor layer-to-layer adhesion, printed parts are more often used as prototypes than functional materials. The next generation of AM utilized vat polymerization of resins in processes such as stereolithography (SLA) and digital light processing (DLP). These reaction-based printing processes initially appeared to inherently overcome the layer-to-layer adhesion issues that were apparent because vat polymerization covalently bonds each of the printed layers. However, the parameters utilized during the vat polymerization has been observed to effect the mechanical properties of the 3D printed part. Each parameter, such as print direction, layer thickness, exposure time, and post-processing, has been previously observed to impact the bulk properties, but the vat polymerization process often is still treated as a ‘black box.’<br/>Utilizing the knowledge gained from decades of studying photopolymerization and cross-linked polymer networks, this work aims to shed light into the ‘black box’ that is 3D vat polymerization. Some of the fundamentals of polymer science have taught the community that structure of the cross-linked network dictates the bulk material properties, and that the structure is dictated by the kinetics of the polymerization. The kinetics of vat polymerization are significantly impacted by the printing parameters such as layer thickness as well as components of the resin formulation such as reactivity of the monomers and geometry of the dye. Through a targeted study, we aimed to decouple the effects each printing parameter had on 3D printed material. The learned relationships connect well-established polymer science knowledge to the newly established field of AM such that printing different resin formulations is less of a ‘guess and check’ process, and the bulk properties of printed parts are more predictable.