Additive manufacturing of thermoset polymer resins requires some mechanism for solidifying deposited layers during the printing process. This can be solved with changes to the polymerization chemistry; however, in order to print legacy thermoset resins such as epoxy or silicone, chemistry cannot be modified and a method of in-situ heating is needed which can provide rapid and non-contact temperature control. To accomplish this, non-equilibrium atmospheric plasma is proposed for its ability to target, heat, and cure conductive composites. This work first investigates the mechanism through which dielectric barrier discharge (DBD) heats an epoxy / carbon nanotube (CNT) composite under atmospheric conditions. Plasma applied to resin surfaces is found to cause rapid heating, with heating rate controlled by adjusting the applied power. Heating is localized to within the top 0.5 mm of the sample surface and the maximum temperature is found to depend on sample conductivity, indicating the heating reaction occurs through a combination of both electron conduction and ion bombardment. Characterization of composites cured using plasma shows both oxidation and roughening of the composite surface, based on spectroscopy, calorimetry, and microscopy. Changes to surface chemistry are expected for a plasma system, and either mitigation or intensification of surface changes could be manipulated through changes to the plasma or atmosphere.
With an understanding of the impacts and mechanism of DBD heating, DBD can then be applied for in-situ additive manufacturing. This was demonstrated via development of a combined device, consisting of a direct ink write (DIW) printer and a DBD plasma. This technology can induce non-contact, volumetric heating within deposited epoxy / CNT resin while simultaneously extruding new material, and parts were printed up to the maximum height of the printer setup (14 cm). Complex structures such as overhangs and bridges were demonstrated, with a greater degree of cure in deposited layers found to enable steeper angles and longer bridges. Imaging revealed high porosity in printed parts, and mechanical testing showed a decrease in the strength of DIW-DBD printed parts when compared to molded. This combined work both demonstrates the functional applicability of DIW-DBD, while also characterizing the impacts of plasma-heating on thermoset resins. Future work can find ways to mitigate porosity caused by this technique, either through adjustments to printing parameters or by changing the plasma characteristics.
