Jetting stability of molten caprolactam in an additive inkjet manufacturing process

This paper proposes a novel additive manufacturing concept which uses inkjet technology to manufacture parts in nylon 6. The methodology involved a series of experiments to investigate whether caprolactam, the monomer of nylon 6, could be jetted in the molten state. Once this was established, further experiments were undertaken to determine the possible boundaries for the process parameters. The main parameters were the vacuum level at which the melt could be controlled for stable jetting, the jetting voltage amplitude and frequency which were investigated against the jet array stability by monitoring the printhead’s nozzle plate. The jet(s) instability behaviour were characterised in different sets of experiments in order to optimise the jetting conditions for the molten caprolactam. The jetting voltage and vacuum level were found to have a significant effect whereas the jetting frequency did not. Instabilities occurred in the form of individual deviating jet trajectory and also jet array failures. These were found to be initiated by air motion when using inappropriate jetting voltage resulting in incorrect jet trajectory. A combination of incorrect jetting voltage and vacuum level led to air bubble entrapment during jetting and, therefore, rapid failure of the jets. Analysis of jet array instability led to a stable process window for further stages of the research on the additive inkjet manufacturing process development.