Fundamental investigation on inkjet printing of reactive nylon materials

Several additive manufacturing processes have been developed for plastic parts. However, there is an ongoing interest to increase the functionality of these parts which are mainly considered as prototypes due to the material and process limitations. This research investigated a novel additive approach for producing a functional engineering plastic, nylon 6. The idea was to combine inkjet printing technology and anionic polymerisation of caprolactam by depositing mixtures of caprolactam with activator and catalyst on top of each other under the appropriate conditions. An experimental setup was integrated based on two identical jetting assemblies with pneumatic and thermal control, synchronised with a deposition system for the reaction of the mixtures upon radiation heating. Different offline material characterisation and inline process monitoring methods were employed to obtain an understanding of the material behaviour at each stage of the research. These included the use of high speed imaging, fluorescent microscopy, particle tracking and image analysis tools. Samples were monitored before and after the drop-on-drop deposition and radiation heating, and then assessed by thermal analysis to find the appropriate conditions for the reaction. It was found that although some monomer conversion was achieved, the rates were much less than with the bulk polymerisation approach. Jetting of thousands of tiny droplets in air could have resulted in a very high monomer deactivation. This highlighted the importance of the environment as a more significant parameter for jetting of nylon 6 compared with the conventional method.