Effect of Post-treatment Process on the Mechanical Properties of 3D Printed Natural Fiber Composite Materials

In recent years, composites based on renewable reserves have increased involvement to reduce the environmental impact and petroleum utilisation. Among the biobased polymers, polylactic acid (PLA) is important thanks to its high versatility and different cheap commercial grades. However, its intrinsic characteristics often reduce its practice in principal applications and some strategies, such as using additives, are adopted to improve some mechanical features [1, 2]. Flax, hemp and jute are the organic fillers greatest used to manufacture green composites, significantly improving their mechanical performance compared to neat matrices [3,4,5,6]. Elsewhere in the vegetable fillers, basalt shows an interesting renewable quality, it is quite economical, derives from a vulcanic rock and shows good mechanical and thermal features [7]. However, the level of the enhancements reached depends on several aspects, such as the chemical-physical features of initial raw materials ma, the filler distribution, the fibers orientation and manufacturing technologies [8,9,10]. In this frame, the probability of achieving product customization is increased with the spread of the fused deposition modelling (FDM) technology. The last is an economic and time effort additive technology to produce complex structures using filaments with several filler concentrations [10,11,12,13]. FDM is an auspicious technology in diverse fields, being also able to comply with the standards of economic sustainability that are more and more required in the industrial sector [14,15,16,17,18,19]. However, also the aesthetic and operational quality of the final components is affected by several factors [20,21,22,23,24]. Even if short natural fiber could increase the mechanical features of the additive structure, poor interfacial adhesion is a very crucial matter [9]. The results disclose that fiber length improves mechanical properties even if, as the length of the fibers increases the flexural strength of the component decreases. This effect can be correlated to growth in defects in the filling layer as well as to poor adhesion between the overlapping layers during the components manufacturing procedure. Therefore, the mean length and content of fibers have to be enhanced to reach satisfactory results. On the other hand, the mechanical features of biodegradable polymers and, in general, semicrystalline thermoplastics depend on the crystallinity degree [25]. The crystallization of a thermoplastic polymer is correlated to its microstructure and this behaviour can be highlighted in FDM, where the molten polymer extruded through the heated nozzle fuses with the material in the prior layer and instantly cools down forming a metastable phase where the crystallization potential of the material is not exhausted. To complete the crystallization of PLA a subsequent heat treatment is required [26,27,28,29,30]. In this work, the selected fillers are basalt powder with average dimensions of less than 50 µm, and short hemp fibers, with an average length equal to 85 µm and a section of less than 50 µm. The products prepared by FDM with different filament raster angle deposition were compared in terms of mechanical performance evaluated with the help of bending tests and Charpy tests. The main aim of the research is to evaluate the innovative material, the effect of filling on the printability of the selected compounds and on the performance of the final component as well as to analyse the effect of the heat treatment on the mechanical features.