Melt electrowriting (MEW) is an additive manufacturing (AM) technology that accurately fabricates polymer fibres onto a collector. The characteristic feature of the technology is a strong electric field between the nozzle and the collector that allows to achieve microscale resolution of the fibre, from 0.8 to 50 microns. With the nozzle raised above the collector, the material deposition process can be accurately monitored using a camera. Machine vision (MV) is a method that enables real-time monitoring and analysis of a process using a camera. Recognizing the successful implementation of MV in metal AM[2,3], here the concept was applied to MEW to analyse the stability of the printing process by measuring three process signatures (Taylor cone area, jet angle, fibre diameter) across three different electric field environments (decreasing, increasing, constant) and control with two air pressure levels (0.4 and 0.7 bar) for each group.
A custom-built MEW printer with both flat and tubular collectors was operated to perform printing experiments. Medical-grade poly(ε-caprolactone) was used to fabricate a continuous “U” shaped printing path to capture fibre pulsing with a high-resolution camera (Alpha 7 III, Sony Corporation, Japan) and telescopic lens (ED AF Micro NIKKOR 200 mm lens, Nikon Corporation, Japan). For experiments where the parameters were adjusted during the process, the change in voltage or collector distance was made for each printed layer. An image analysis algorithm developed in MATLAB was used to measure Taylor cone size and the angle of the jet flight from the images captured with the camera. Fibre diameter was measured using SEM (Carl Zeiss Microscopy, Göttingen, Germany).
It was shown that all process signatures reveal information about the stability of the printing process. When the collector distance is increasing during the print, the process signatures commence an oscillatory behaviour due to a decrease in the electric field over time indicating unstable printing conditions. In a constant electric field environment, however, the decreasing electric field is compensated by incrementally increasing high voltage which results in a stable fabrication process. The new knowledge about the stability of the process was translated to the mandrel collector to print 150-layer high-quality tubular scaffolds. Additionally, it was demonstrated that the Taylor cone area provides more accurate and more reliable information about printing stability and fibre diameter during the print when compared to the jet angle.
The Taylor cone volume was identified as an important process signature for process quality identification in MEW. MW was applied to analyse the stability of the printing process in real-time. The investigation allowed to gain a better understanding of the effect of the electric field on the quality of the printed fibres. The findings were used to manufacture thicker layer constructs. This work demonstrates the importance of real-time monitoring in multiparametric systems such as MEW.
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