Issue 48

C.M.S. Vincente et alii, Frattura ed Integrità Strutturale, 48 (2019) 748-756; DOI: 10.3221/IGF-ESIS.48.68 749 K EYWORDS . 3D printing; PLA; mechanical properties; water absorption; printing parameters I NTRODUCTION n recent years the use of 3D printing technologies has increased significantly and this rapid growth it is expected to remain in a near future. The 3D printers are now commonly used for the manufacture of a broad array of products, ranging from leisure articles to medical components [1]. From all available 3D printing technologies, one of the most popular to the public is fused deposition modelling (FDM) due to the vast number of companies that develop and market 3D printers based on this technology, at relatively low cost [2]. FDM printed materials include among others : acrylonitrile butadiene styrene (ABS), poly – lactic acid (PLA), polycarbonate, nylon and polyamide, which are now available for building mechanical parts with diversified characteristics, on demand [3]. From the listed materials PLA, a biodegradable thermoplastic polymerized from lactic acid from natural sources such as corn [4], was one of the materials that attracted most attention due to their unique properties for 3D printing. The main advantages of PLA are low printing temperature, smoother appearance, high geometric resolution, low warping effect and biodegradability. On the other hand, the comparative disadvantages with other thermoplastics relies on his brittleness and low thermal stability. As 3D printing technology spreads into more domestic users or at industrial level, PLA emerge as a valid sustainable thermoplastic alternative to non–biodegradable polymers, able to address the problem of residues derived from manufacturing processes, which impacts the environment of our planet. In this context, the characterization and improvement of the mechanical properties of PLA parts produced by FDM it is a topic of great interest, so that this material can be used on a reliable way. On the past several studies stablished a relation between FDM parameters and their combinations on the mechanical properties of 3D printed PLA [5–11]. These studies allow the designer to define the 3D printing conditions (so that they can meet mechanical design requirements) by controlling the strength and ductility of materials, or assigning variable mechanical properties to parts, according to their final application. From the literature it’s well know that the higher strength of PLA parts produced by FDM is linked to higher extrusion temperatures [12–14], raster angles aligned with the direction of the applied force [7,9,15] and smaller layer thicknesses [7,8,16], while the ductility follows the inverse trend. These trends of the FDM process provide guidelines for the definition of processing parameters of PLA parts regarding his application in elements of strength or ductility/toughness, depending on the project requirements. Another important issue that also must be considered on the project of PLA components by FDM is water absorption, especially when exists direct contact with fluids, such as in the case of underwater [17], biomedical [18], or microfluidic devices [19]. Water absorption can arise due to the porosity of the printed part or from the PLA material itself. It is known that the polar bonds in PLA can make it susceptible to water absorption, which can cause partial breakdown the PLA polymer chains and change mechanical properties, turning PLA even more brittle. Moreover, after the 3D printing process PLA becomes more chemically reactive, increasing the susceptibility of PLA to water,[20] which will tend to decompose the material over time [21,22]. A classical, simple and inexpensive way to protect a water reactive material is by applying a protective coating using the painting method. This method was successful applied on surface modification of ABS reducing water absorption (from 1.6 to 0.4 % in weight) [23] and on the protection of a PLA hull with a polyurethane (PU) coating, reducing water absorption from 3.8 to 1.5% in weight [17]. Despite the significance of this topic, there is also a lack of studies evaluating the impact of surface treatments on mechanical properties of FDM parts made with PLA, with most works mainly focused on evaluating the staircase effect reduction [24–27]. The few reported works evaluating the impact of surface treatments on mechanical properties of PLA parts produced by FDM, are until now limited to blow cold vapor surface treatments [28]. On our work, PLA specimens were fabricated by FDM with different levels for the 3D printing parameters: layer thickness, raster angle and extrusion temperature, aiming to define distinct levels of strength (improved ultimate tensile stress, modulus of elasticity and yield strength) and ductility/toughness (improved fracture strain and toughness). The 3D printing conditions of specimens were replicated on cubic samples that were subjected to water absorption tests. The water absorption was estimated for uncoated and coated samples with acrylic or polyurethane (PU) protective coatings. The influence of protective coatings on mechanical properties were also evaluated trough tensile tests. The use of protective coatings for watertight applications of PLA 3D printed parts, will enable new designs where both sealing and the preservation of mechanical properties are critical aspects. To the best of our knowledge the water absorption I