As biodegradable thermoplastics are more and more penetrating the market of filaments for fused deposition modeling (FDM) 3D printing, fillers in the form of natural fibers are convenient: They have the clear advantage of reducing cost, yet retaining the filament biodegradability characteristics. In plastics that are processed through standard techniques (e.g., extrusion or injection molding), natural fibers have a mild reinforcing function, improving stiffness and strength, it is thus interesting to evaluate whether the same holds true also in the case of FDM produced components. The results analyzed in this review show that the mechanical properties of the most common materials, i.e., acrylonitrile-butadiene-styrene (ABS) and PLA, do not benefit from biofillers, while other less widely used polymers, such as the polyolefins, are found to become more performant. Much research has been devoted to studying the effect of additive formulation and processing parameters on the mechanical properties of biofilled 3D printed specimens. The results look promising due to the relevant number of articles published in this field in the last few years. This notwithstanding, not all aspects have been explored and more could potentially be obtained through modifications of the usual FDM techniques and the devices that have been used so far.

The use of wood fibers is a deeply investigated topic in current scientific research and one of their most common applications is as filler for thermoplastic polymers. The resulting material is a biocomposite, known as a Wood Polymer Composite (WPC). For increasing the sustainability and reducing the cost, it is convenient to increase the wood fiber content as much as possible, so that the polymeric fraction within the composite is thereby reduced. On the other hand, this is often thwarted by a sharp decrease in toughness and processability—a disadvantage that could be overcome by compounding the material with a toughening agent. This work deals with the mechanical properties in tension and impact of polypropylene filled with 50 wt.% wood flour, toughened with different amounts (0%, 10%, and 20%) of a polypropylene-based thermoplastic vulcanizate (TPV). Such properties are also investigated as a function of extrusion processing variables, such as the feeding mode (i.e., starve vs. flood feeding) and screw speed. It is found that the mechanical properties do depend on the processing conditions: the best properties are obtained either in starve feeding conditions, or in flood feeding conditions, but at a low screw speed. The toughening effect of TPV is significant when its content reaches 20 wt.%. For this percentage, the processing conditions are less relevant in governing the final properties of the composites in terms of the stiffness and strength.

Fused-filament fabrication is one of the most popular 3D printing techniques for thermoplastic materials because it is easy to use and is low-cost. On the other hand, it has the great limit of being suitable only for developing prototypes, because the printed object generally has low mechanical properties, and this prevents its use in structural applications. To solve this issue, the scientific literature has mainly focused on the optimization of the printing parameters and on performing some post-printing treatments, e.g., annealing, but despite some results being very promising, the topic has not yet been exhaustively investigated. In this paper, a post-printing treatment was studied that was based on two subsequent stages of remelting and compaction within a mold made of a granular material. The material chosen for this study was a green composite made of poly-(lactic acid) and poly-(hydroxyalkanoate) filled with wood fibers. The density, mechanical properties in terms of tension and microscopic observations were used to evaluate the treatment effectiveness. The main results were that voids were reduced, and the quality of the interlayer welding was increased, and this improved the mechanical properties, both in terms of stiffness and strength. In particular, the initial specimens displayed remarkable anisotropy, being stronger and stiffer in the longitudinal direction. After the post-processing treatment, despite the longitudinal properties having a very limited increase, the transversal properties increased significantly until they reached the longitudinal properties, thus leading to a more isotropic material.

Purpose The architecture of 3D-printed parts made through fused deposition modelling (FDM) with raster infill resembles that of composite laminates. Classical lamination theory (CLT), the simplest model for composite laminates, has been proved successful for describing the stiffness properties of FDM parts, while strength modeling so far has been limited to unidirectional lay-ups. The aim of this paper is to show that CLT can be used to predict also FDM part failure. Design/methodology/approach Wood flour-filled polyester has been chosen as a model material. Unidirectional specimens oriented at 0°, 90° and ± 45° have been first characterized in simple tension to obtain the properties of the single layer. Next, two quasi-isotropic lay-ups, possessing different layer sequences, have been tested again in simple tension for CLT validation. Findings The measured properties are in good agreement with theoretical predictions, both for stiffness and strength, and an even better agreement can be achieved if a correction for taking the contour lines into account is implemented. Originality/value The paper shows that also the tensile strength of FDM parts can be predicted by using a mathematical model based on CLT. This opens up the possibility of using CLT for studying optimization of raster filled lay-ups, for example in terms of the best raster angles sequence, to better resist applied external loads.

Wood polymer composites or WPCs are increasingly used as substitutes for natural wood in outdoor applications due to their better environmental sustainability and the consequent reduction in carbon footprint. In this paper, the presence of an elastomer used as a toughening agent (Santoprene by Exxon Mobil) in a polypropylene-based WPC containing 50 wt % wood flour was investigated in terms of its tribological behavior by dry sliding wear tests. These were performed after two environmental pre-conditioning treatments, i.e., drying and water soaking. The ball-on-disk configuration under a constant load was chosen along two sliding distances. Dynamic mechanical thermal analyses were used to reveal the effect of the toughening agent on the storage modulus and damping factor of the composites. Results in terms of weight loss measurement and coefficient of friction were obtained, together with surface morphology analysis of the worn surfaces at the scanning electron microscope and 3D profilometer. An abrasive wear mechanism was identified, and it was shown that the toughening agent improved wear resistance after both pre-treatments. This beneficial effect can be explained by the increase in strain at break of the WPC containing the elastomer. On the other hand, the water soaking pre-treatment produced severe damage, and the loss of material cannot be completely compensated by the presence of the toughening agent.

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Wood Polymer Composites are materials that are more and more used and studied by both academic and industrial research. Although such materials are successfully substituting natural wood in many relevant applications, still some of their drawbacks limit their widespread diffusion. One of these is their brittleness, which can be limited by blending with a toughening agent. In this study we investigated the rheological properties, in terms of the flow curve and the wall slip characteristics, of a toughened Wood Polymer Composite. Testing was performed using an in-line slit rheometer attached directly to a single screw extruder. The material used for the experiments was a blend of polypropylene-based wood polymer composite compound, containing 35 wt.% of wood flour and a suitable thermoplastic elastomer used as a toughening agent. The results showed a shear thinning behaviour and a significant slip velocity of the material that is beneficial during processing.

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