A new material class for the most widely used additive manufacturing process S. Baumann, B. Ulmer, M. Lang, T. Hochrein, M. Bastian, O. Adrian, G. Körber The use of extrusion-based 3D printing in both industrial and private areas has grown significantly and the range of processible materials was considerably broadened. However, TPS have still not been available for this additive manufacturing process. Within the scope of a research project, TPS grades with hardness down to 60 Shore A were developed, opening up new application areas for extrusion-based additive manufacturing from soft and elastic printed parts to two-component objects with hard/soft combination. Introduction Additive manufacturing, also known as 3D printing, designates a group of fabrication technologies working on a layer-by-layer assembling process. 3D printers allow a tool-free production of parts directly from three-dimensional computer aided design (CAD) models, thus offering great advantages in terms of time and cost saving and geometric freedom. For the past 30 years additive manufacturing has been used for the fabrication of functional prototypes and visualization models (rapid prototyping) as well as for the manufacturing of tools (rapid tooling) and is gaining increasing importance in the field of end-use part production (rapid manufacturing) now. The main applications are found in the sectors of machine engineering, aerospace and automotive industries, consumer products and electronics as well as in the medical and dental fields. In 2016 the additive manufacturing industry grew by 17.4 % to 5.62 billion euros. 80 % of the service providers are dealing with additive manufacturing machines working with polymers [1]. Fused Layer Modeling (FLM) or Fused Filament Fabrication (FFF) is a well-known extrusion-based additive manufacturing process working with polymers. FLM/FFF printed products are generated by feeding a filament of thermoplastic material through a heated extrusion nozzle which deposits material strands next to or upon each other from bottom to top on a building platform. Because the equipment required is comparatively cheap and easy to handle, the use of FLM/FFF in both private (decorative elements, toys, spare parts) and industrial areas (automotive and aerospace sectors, machine engineering, medical technology) has grown significantly and the range of processible materials was considerably broadened. However, only a few soft and flexible filaments are available, which are made up of urethane based TPE (TPU) and ester/ether based TPE (TPC). Styrene based TPE (TPS), which represent the most widely used TPE class in the world [2], have still not been available for this additive manufacturing process. Due to its interesting properties (wide hardness range, good ageing and weathering resistance as well as good adhesion to polyolefins like polypropylene), TPS is often used for technically challenging applications in the automotive, electrical and construction sectors and in household products, as well as for multi-component parts e.g. in hard/ soft combinations like grips and sealing elements. Within the scope of a research project, the research institute SKZ together with the filament producer Herz and the engineering office Achatz & Grauel supported by the TPE compounder Allod Werkstoff GmbH & Co. KG, Burgbernheim, Germany, develop TPS for the use in FLM/FFF process. Recipe, compounding, filament extrusion and FLM/FFF printing processes have to be optimized for this new material class. Production of TPS compounds and filaments The TPS compounds consisting of different styrene block copolymers, PP and plastizisers in variable amounts were prepared on a co-rotating twin screw extruder in a single-step extrusion process. The compounding parameters were adjusted to the new TPS recipes. For the production of the filaments, which are required for the FLM/FFF process, an extrusion line composed of a single screw extruder, a redesigned die, a cooling unit, a pull-off device and a wind-ing machine was optimized. Filaments with both standard diameters of 1.75 mm and 2.90 mm were extruded (fig. 1). A laser measuring system assured the online monitoring of the filament diameter. The TPS filaments produced show better diameter accuracy than the usual ± 0.05 mm (fig. 2). Extrusion-based 3D printing of TPS The TPS filaments were processed on a commercially available FLM/FFF 3D printer, which was upgraded with an adequate filament feeding system, so-called direct drive extruder, to prevent from the buckling issue occuring in the printing head when printing flexible materials. In contrast to stiff filaments flexible filaments can deform between the drive gear driven by a stepper motor (so-called cold end) and the heated liquefier unit in front of the nozzle (so-called hot end). Therefore the pressure built up by the driven gear on the filament cannot be transmitted to enter the hot end and the material cannot be printed. After optimization of the feeding mechanism the TPS filaments could be processed on the FLM/FFF 3D printer. Tensile bars of type S2 according to DIN 53504 were printed and analysed to estimate the mechanical properties of extrusion-based 3D printed TPS (fig. 3). Results The developed TPS filaments can easily be processed in the FLM/FFF 3D printer. They show no stick-slip effect and have a very good adhesion to the building platform, even without heating the printing bed. A printing speed of at least25 mm/s can be reached. With 1.75 mm filaments TPS materials with a hardness down to 75 Shore A can be printed. With2.90 mm filaments even softer materials with 60 Shore A are processible. Besides a high flexibility and elasticity, FLM/FFF 3D printed parts made up of TPS show high mechanical properties. Tensile strengths of7.7 MPa and 10.7 MPa and elongations at break up to 626 % and 605 % have been measured on printed TPS tensile bars with 75 Shore A and 60 Shore A respectively (fig. 4, 5). Summary and outlook TPS have been successfully developed for the use in the most widely used additive manufacturing process working with filaments. TPS grades with hardness down to60 Shore A, tensile strength up to 10 MPa and elongation at break above 600 % could be printed with a FLM/FFF 3D printer. The new TPS open up new application areas for extrusion-based additive manufacturing from soft and elastic printed parts like bellows (fig. 6) to two-component objects with hard/soft combination. The project team is now working on the development of filaments made up of compatible hard materials like polypropylene. Acknowledgements The project is funded by the Federal Ministry for Economic Affairs and Energy (BMWi) according to a decision of the German Federal Parliament. The authors thank for the support. References [1]T. Wohlers, Wohlers Report 2017 – 3D Printing and Additive Manufacturing State of the Industry, Annual Worldwide Progress Report, 2017. [2]Ceresana, Thermoplastic Elastomers, Market Study, 2015. Source:TPE Magazine