J. Gonzalez-Gutierrez, L. Hentschel, S. Petersmann, M. Shivokhin
Luxembourg Institute of Science and Technology,
Keywords: copolymers, additive manufacturing, material jetting
Summary:Thermoplastic elastomers (TPE), as the name implies, are polymeric materials with properties allowing them to be melt-processed and reprocessed while having an almost instantaneous, linear, and reversible elastic response to high strains at service temperatures. These characteristics make TPE suitable for the manufacturing of soft products by traditional forming techniques and recently additive manufacturing (AM) techniques such as powder bed fusion (PBF), material extrusion (MEX) and material jetting (MJT). Among AM technologies, MEX with filaments, also known as fused filament fabrication (FFF), is one of the most widely utilized technologies due to the low-cost equipment and simplicity of operation. However, the need to prepare filaments with sufficient stiffness limits the type of TPE that can be processed. As a result, alternative AM technologies have been invented and commercialized. One example of these newer technologies is the Arburg Plastic Freeformer (APF). This MJT technology relies upon the plasticization of thermoplastic pellets in a device similar to an injection moulding machine. However, instead of injecting the plasticized thermoplastic into a mould, it is deposited as droplets on a build platform to build three-dimensional objects layer-by-layer. In this work, three propylene-ethylene copolymers TPE were selected to assess their suitability for FFF and APF. The three copolymers had the same melt flow index of 8 g/10 min, but they had different ethylene monomer contents of 4, 9 and 11%, and as a result, their shore hardness was 50D, 34D, and 27D, respectively. Filaments with a 1.75 mm diameter and a direct extrusion FFF machine were used. It was observed that FFF could successfully process only the hardest copolymer due to the low stiffness of the other filaments that led to buckling in the filament feeding area. On the contrary, all three grades of TPE could be successfully processed by APF since the pellets can be successfully plasticized and deposited into the build platform. Three different APF parameters were investigated (i.e., form factor, temperature chamber and build orientation) to maximize the mechanical properties beyond the specimens prepared by FFF for the copolymer with the highest hardness.