The FLM process offers the possibility of functionalizing components by introducing continuous fibre reinforced structures and thus the tool-less production of fibre reinforced composite components. The high development requirements for printing material, printhead, process maturity, process stability, design specifications and design notes represent a high hurdle for greater market penetration and acceptance. In particular, the development of suitable continuous fibre reinforced printing filaments remains a component of current research projects. Overcoming these hurdles and therefore opening up a broader field of application for continuous fibre reinforced additive manufacturing by developing of robust printing filaments is the main goal of the FIBRE in the joint project FIONA (Function-integrated optimized new types of additive structures).
It was determined that instead of impregnating the fibres in the printhead a pre-impregnation must be applied in a separate process to ensure sufficient material quality. Such a process was developed in the joint project FLATISA (Flame-retardant, temperature-resistant thermoplastics for industrial series use of additive manufacturing processes). The roving is unwound from a coil, preheated by a fibre oven and a heating tile, and then pulled through an impregnation tool. A single-screw extruder (Collins) feeds the melt of the thermoplastic material into this tool. The roving is impregnated in the tool cavity, dispensed through the outlet nozzle as the printing filament and wound onto another spool.
The tests were carried out with flame-retardant polyamide polymer as well as different types of carbon fibre rovings. Nozzle diameters from 1.5 - 0.7 mm were used to set specific fibre volume contents in the range of 35 - 45 %. The process optimisations have resulted in a stable process, a more homogeneous distribution of the individual filaments and a significantly rounder filament with fewer defects. However, additional adjustments are necessary to further improve the filament quality in follow-up project FIONA.