Challenges in drop-on-drop deposition of reactive molten nylon materials for additive manufacturing

An inkjet-based additive manufacturing process must overcome a variety of challenges including uniform layer fabrication, especially when the process involves deposition of multiple materials in each layer. Research is being conducted into a novel process to synthesise nylon 6 via inkjet printing of two reactive mixtures consisting of molten caprolactam with an activator and a catalyst. Jettability of the materials has been reported while this paper presents the results of drop-on-drop deposition of the molten materials. Droplet/surface interactions, formation and stability of beads, droplet placement accuracy and drop-on-drop mixing are reported. Results show that although spreading occurred during impingement, the first five droplets produced a bulge at the start of the bead. The bead was then observed to be uniform with further droplet impingement on heated and cold surfaces. However, it did not retain the stability on a heated surface and bulges and ridges developed within a second. Drop-on-drop placement was not accurate when using the catalyst mixture. This was because the undissolved catalyst complex (in form of microcrystals) agglomerated and it was possibly this that disturbed the meniscus at the nozzle and led to a jet trajectory error. Repeated drop-on-drop deposition confirmed that the impingement kinetics provided mixing which is crucial to produce nylon.