Material and process engineering aspects to improve the quality of the bonding layer in a laser-assisted fused filament fabrication process

The mechanical properties of components manufactured ("printed") by the Fused Filament Fabrication (FFF) process are usually limited by the cohesion achieved between melt-processed thermoplastic filament strands perpendicular to the filament deposition direction. This significantly limits the usefulness of the FFF process and commercial FFF printers for the production of a wide range of functional components and consumer goods, so that the FFF process is currently used for the production of prototypes for visualization purposes only. In order to open up potential markets for structural components in the future, the realization of an optimized bonding layer quality via a significantly improved cohesion between the filament strands is essential. Since the mechanical properties of polymeric materials generally depend on the density of molecular entanglements and tie molecules in the material structure, it is of the utmost importance to realize the highest possible density of such entanglements and tie molecules between the individual filament layers also in FFF components. The present work deals with different methods of heat input to improve the entanglement and tie molecule density in the bonding layer of FFF components through enhanced polymer diffusion at temperatures far above the glass transition temperature for amorphous plastics or the melting temperature for semi-crystalline plastics, respectively. Heat can be either globally or locally introduced by conduction, convection, and radiation, the latter being the most effective for the FFF process when applied locally via properly controlled laser radiation. Ideally, a compact, cost effective and easy to use radiation source is used for this purpose.Hence, a setup based on a diode laser is presented in this work. Furthermore, an idealized calculation of the amount of heat required to promote the formation of interfilament bonds and thus to achieve an increased mechanical property profile was carried out. Two types of materials commonly used for the FFF process, polylactic acid (PLA) and acrylic-butadiene-styrene copolymer (ABS), were used for the experimental procedure. After producing a special monofilament with improved laser absorption characteristics, test specimens were produced. As a reference, test specimens made of the same materials were printed with the conventional FFF process. The differently manufactured test specimens were characterized optically and visually, using a stereomicroscope, and mechanically, using tensile tests. In addition, specimens were produced with a specially designed device for local heat input onto the substrate layer region just ahead of the new filament deposition. The effectiveness of this laser-assisted method could be proven in tests by adjusting the process parameters of the FFF process. After finding the optimum printing speed (equivalent to the laser power applied) for the materials PLA and ABS, an optical examination of the fracture surfaces of test specimens with filaments laid down perpendicular to an applied tensile load showed a significant increase in plastic deformation capacity, indicative of an improved interfilament bonding. This was confirmed by the mechanical property values determined in tensile tests. Thus, for optimized printing speeds, an increase in tensile strength values of similar to 20% for PLA and similar to 7% for ABS was achieved. For elongation at break, the improvements were similar to 21% for PLA and similar to 34% for ABS, respectively. The methodology of laser-assisted FFF printing thus offers great potential for mechanically and structurally optimized functional components with specifically introduced anisotropy.