PSI - Issue 42

Ralf Lach et al. / Procedia Structural Integrity 42 (2022) 3–8 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Additive technologies for fast production of prototypes, the so-called Rapid Prototyping, were used since the late 1980s. Recently, within the scope of Rapid Manufacturing, selected additive technologies have become increasingly more important for the fast production of functional parts directly usable by the end costumer. Currently there are three different groups of additive technologies which are, in principle, of particular importance for Rapid Manufacturing of plastic parts: liquid-based technologies such as stereo lithography, powder-based technologies such as selective laser sintering, and extrusion-based technologies such as Fused Filament Fabrication (FFF) or the ARBURG Freeformer technology, the last two often subsumed as 3D printing. An overview of the additive technologies is given by Gebhardt (2016, 2016a), Grießbach (2015, 2016) and Gibson (2010), for example. Compared to conventional processes such as injection moulding, layer-based technologies offer various advantages. Due to the layer structure there are less limitations in design which opens up a broad spectrum of design possibilities. This enables manufacturing of complex functional systems in one piece without subordinated montage. Another very important point of these technologies is that tooling costs (as required for injection mouding) can be omitted, which become economically interesting in the case of special purpose machinery manufacture, the medical sector as well as the aerospace industry (Sauer, 2005; Grießbach, 2012). A fundamental disadvantage of all these additive technologies is, however, that they are often strongly limited to special groups of polymer materials. Also the portfolio of recently FFF-printable polymers is still limited to few amorphous or slightly crystalline polymers, such as acrylonitrile – butadiene – styrene copolymer (ABS) investigated in this study. Determination of the mechanical properties is generally based on uniaxial tensile testing only. Investigations at impact loading by means of the impact test for notched or unnotched specimens, for example, are rather the exception (Grießbach, 2010, 2012; Kaut, 2018; Mautner, 2016; Es-Said, 2000; Roberson, 2015; Wang, 2017; Tsouknidas, 2016). As a result, a linear relationship between the mechanical properties, such as the notched impact strength, and the density or the amorphous fraction, respectively, for various polyamide materials was found (Grießbach, 2010, 2012). Also investigations dealing with the crack propagation and toughness are rather scarce in the case of plastics produced via Rapid Technologies (Grießbach, 2012; Kaut, 2018; Lach, 2018; Blattmeier, 2012; Van Hooreweder, 2013; Aliheidari, 2017; Hart, 2017; Arbeiter, 2018). FFF as example for 3D printing is characterized by distinctly pronounced material porosity compared to optimized laser sintering. Thus, 3D printers were primarily configured mainly in respect of good precision in printing, high printing speed and low cost. The mechanical properties, which scatter within a wide range for a given material, were considered less meaningful in the past (Grießbach, 2015, 2016), at best they were mostly documented for tensile loading (see Cuan-Urquizo (2019) and references therein). In summary it can be concluded that fracture mechanics investigations of additively manufactured plastics are hardly known. 2. Materials and Specimen Preparation Two amorphous thermoplastic polymers were selected to be investigated: acrylonitrile – butadiene – styrene copolymer (ABS) and polycarbonate (PC); for Fused Filament Fabrication (FFF), i.e. 3D printing, the materials were provided in form of filaments. Compression-moulded plates were prepared based on shredded filaments for comparison. For specimen preparation a 3D printer of the type HT500.2 manufactured by Kühling&Kühling GmbH, Kiel (Germany) was used. Within the first step, based on standard printing parameters the processing parameter of the printer were optimized for the model material ABS as an essential precondition of reproducible specimen preparation. To verify the level of optimization by systematic variation of printer-specific parameters such as layer thickness, overlap, extrusion width etc. the porosity and the mechanical properties from uniaxial tensile test are taken into account. Setting these optimized basic processing parameters as a constant, further processing parameters were selected to be varied within the subsequent investigations. Both building orientation (45°/45° and 0°/90°) and printing speed (60 and 80 mm/s) were considered specially promising. The thickness of all printed specimens was