Printing process and characterization of fully pad printed electroluminescent panels on curved surfaces

Precision printing of particulate slurries with particle sizes of 5–20 µm using an indirect gravure printing, specifically with pad printing technology, offers interesting applications of functional printing on 3D surfaces. We present our results for printed electroluminescent panels (EL), where printing inks containing luminescent and dielectric particles of such comparatively huge size are transferred from a gravure printing form with gravure patterns not much larger than the particles. Compared with screen printing, the benefit of pad printing is that fairly complex and corrugated surfaces of 3D printed bodies can be endowed with a particulate functional surface. We demonstrated this using commercially available materials for electroluminescent panels. From the electrical point of view, EL panels are capacitors with a luminescent and insulation layer which enables a careful investigation of the printed surface by the characterization of their electrical and optical performance. We emphasize that the printing process we developed is not restricted to EL panels but could also be used for printing protective, electrochemically active, or radiation absorbing metal or metal oxide coatings to 3D printed fused deposition modeling polymer bodies. Printing parameters such as pad hardness, printing velocities, and pad surface tension are investigated. Further, we consider the relation between the printing process, the thickness of the insulation layer, the volume fraction of particulate BaTiO₃ in this layer, and EL panel luminance. Our pad printed EL panels, using adapted ink formulations, achieved average luminance of 140 cd/m² on curved surfaces which also corresponds to the performance standards of conventional screen printed panels.