Volume 37, Issue 1

Fused Filament Fabrication (FFF) is a widely embraced material extrusion (MEX) additive manufacturing (AM) process to produce complex three-dimensional structures, and it is typically used in the fabrication of biodegradable polymers for biomedical applications. However, FFF as a fabrication process for blended polymeric materials needs to be optimized for enhanced mechanical properties. In this work, biodegradable polylactic acid (PLA)/polyhydroxyalkanoate (PHA) dog-bone and notched specimens are printed to determine optimum printing parameters for superior mechanical properties in FFF additive manufacturing. The effect of layer thickness, infill density, and printing bed temperature on mechanical properties is investigated by employing a design of experiments (DoE) approach using response surface methodology (RSM). Experimental results showed the significance of the opted parameters for mechanical properties of the PLA/PHA blend. Then, optimum values for layer thickness, infill density, and printing bed temperature are identified for tensile and impact strength and an empirical relationship between parameters is formulated for low density and cost-effective fabrication. Finally, the analysis of variance (ANOVA) is performed to check the adequacy of the model for the influence of process parameters and their mutual interactions. The verification experiments validated the adequacy of the proposed model for PLA/PHA blend in FFF additive manufacturing.

Carbon black (CB)-filled rubber composites are widely used for tire tread and other rubber products due to their good abrasion resistance and reinforcement. Numerous studies have been focused on the filler-rubber interaction with the aim of obtaining optimum performances. In order to investigate the effect of modification on properties of diene rubber composites, a modified CB-filled isoprene rubber (IR) composite was prepared with a multi-functional silane coupling agent, 2-aminoethyl-2-(3-triethoxysilylpropyl)aminoethyl disulfide (ATD). This modification significantly enhanced the CB-IR interaction and improved CB dispersion. For the modified CB/IR vulcanizates, the tensile strength, stress at 300% elongation, temperature rise, and dynamic loss coefficient were significantly improved at low ATD dosage (1.5 phr). Such modification provides an effective route to prepare CB-reinforced diene rubber composites with low hysteresis and improved mechanical properties.

This paper considers a model of melt spinning with stress-induced crystallization, where the melt uses the Phan Thien-Tanner model, and the solid adopts the rubber elastic model. We design a computationally efficient algorithm for solving the model. The temperature, radius, and birefringence profile in low-speed and high-speed spun PET fibers were predicted and compared with the published experimental data. The simulation results are consistent with the experimental data from the literature. Then a parametric analysis is conducted to investigate the effects of operating conditions on the spinning dynamics and reveal the relationship between the operating conditions and the final properties. The change of take-up speed has a significant influence on the crystallinity and birefringence of the fiber.

The effect of the amount of 2,2′-Bis(2-oxazoline) on the spectral, thermal, morphological, and tensile properties of polypropylene (PP)/liquid crystalline polymer (LCP)/maleic anhydride grafted PP (PP-g-MAH) blends was investigated. A wholly aromatic LCP having a low melting point, namely a AL-7000 copolyester was used, because it also uses a compatibilizer with a low decomposition temperature. As the amount of 2,2′-Bis(2-oxazoline) increased, the dispersibility of the LCP improved and the tensile properties were improved, but when it was added in excess, a loss in function was observed. In particular, when 0.5 wt% of 2,2′-Bis(2-oxazoline) were added, the tensile strength and elastic modulus were the highest, and cohesive fracture (fracture within the matrix, not at the PP/LCP interface) was observed in the scanning electron microscope image.

Polypropylene (PP) reinforced with 2 and 4 wt% of multi-walled carbon nanotubes (MWNT) were melt-blended in twin screw extruder and then molded by compression or micromolding process. The impact of injection speed on the surface morphology, rheological and tensile characteristics was investigated by using a scanning electron microscope, parallel plate rheometry, and tensiometry. Results showed that the tensile properties of micro-molded specimens were remarkably higher than those of the compression molded sheets. Compared to compression molded sheets, micromolded specimens demonstrated up to 40 and 244% higher tensile stiffness and yield strength, respectively, most likely due to the alignment of polymer chain segments in the flow direction induced during the micromolding process. It was observed that the fast filling speed caused a drop in the tensile properties of the nanocomposites and polymer. Rheological examination revealed that the presence of a rheological percolation network in the nanocomposites produced by micromolding and the fast injection speed was beneficial for establishing the percolated network. Morphological examination revealed that the size of nanotube agglomerations that appeared in micromolded specimens was up to five times smaller than in compression molded sheets and the agglomeration size decreased with the increase of the injection speed.

This work presents the mechanical behavior of self-reinforced composites (SRCs) manufactured and modified via film stacking. For modification, interleaved films made of polypropylene (PP), a thermoplastic elastomer and a polyolefin engage were combined in different ways to induce the elastic modifier into the matrix material. The content of modifier was also varied in two ways. First, the films were produced out of a single material and second out of a compound. So, the same content of modifier was implemented in two different ways. It is shown that, in case of this research, only the kind of modifier and the content but not the way of implementation are responsible for the mechanical behavior of SRCs. It is shown that the modification can adjust the tensile strength, tensile stiffness and impact properties in a broad range. It is also shown that different mechanical properties of the composite can be predicted by a regression model that uses the Shore A hardness and the content of modifier.

We implicitly assess the distributive mixing of generalized Newtonian fluids with shear-thinning behavior in a journal bearing flow geometry. For this purpose, we firstly develop a finite element code to calculate the flow field parameters. Our numerical algorithm splits the viscous stress tensor into arbitrary Newtonian stress and a source term, which grows gradually during the iterative solution. Therefore, we get a better converging solution than the Picard method, especially for highly shear-thinning fluids. Secondly, considering two inert fluids in the mixing domain, we employ a Lagrangian-Eulerian approach to predict the shape of the interface between two fluids. The results of our numerical analysis provide us the required information to evaluate three implicit mixing criteria: the concentration variance, the striation thickness, and the mean strain function. Then we conduct a parametric study to investigate the effects of different parameters (geometry and rheology) on the distributive mixing state. In addition, we discuss which mixing criteria provide a better evaluation for distributive mixing.

The performance of thermoplastic polyurethane (TPU) reinforced with natural fibers can be tailored through a suitable choice of the fibers nature or the type of surface treatment applied to them. The present work deals with the improvement of the interfacial properties of natural fibers, namely wood flour (WF) by the introduction of graphene oxide (GO), which may easily disperse on the WF surface to provide hybrid fibers (WF-GO). The latter were then used as reinforcement of a TPU matrix at different ratios of 1, 3 and 5 wt%. The different samples were characterized by FTIR and RAMAN spectroscopies, XRD, SEM and TGA to confirm the structure, morphology and the thermal stability of the prepared hybrid fibers as well as their composites (TPU/WF-GO). SEM micrographs revealed that the surface treatment applied to WF, the distribution of GO sheets on the fiber interface, and the dispersion of (WF-GO) on the polymer matrix were successfully carried out. The thermal stability of the TPU-base composites increased with the increase of WF-GO content from 325 °C for the pure TPU matrix to 343 °C for the composite reinforced by 5% of (WF-GO). In addition, the results confirmed that the incorporation of GO into WF led to a significant improvement in the mechanical properties of the TPU-based composites, with an improvement in strength from 10.9 MPa to 19 MPa.

The amphiphilic copolymer polyacrylonitrile-co-poly(hydroxyethyl methacrylate) (PAN-co-PHEMA) was readily blended with polyacrylonitrile (PAN) to fabricate a flat-sheet blending membrane through non-solvent induced phase separation (NIPS). In the membrane-forming process, the hydrophilic PHEMA chains are uniformly distributed on the surface, as revealed by the energy-dispersive X-ray tests. The sponge-like sub-layer embedded with droplet-shaped structures is formed at the cross-sections of membranes, because of the high viscosity of the casting solution. With the increase of copolymer concentration, the mean pore size of the blending membranes increases from 26.9 to 99.8 nm, leading to the increase of membrane flux from 93.6 to 205.4 l/(m 2 h). The incorporation of PAN-co-PHEMA copolymer endows the blending membrane with a rough surface microstructure and enhanced hydrophilicity. The rejection ratio of membranes for emulsified pump oil reaches 99.9%, indicating a prominent separation performance. In the cycle permeation experiments, the flux recovery ratio of the blending membranes is as high as 99.6%, which is much higher than those of PAN membrane. The irreversible fouling of blending membranes induced by oil adsorption is alleviated, and converted into reversible fouling, owing to the reduction of the adhesion force between foulant and membrane surface. These results suggest that the anti-fouling property of PAN membranes has been dramatically strengthened via the addition of PAN-co-PHEMA copolymer.

Prior literature employed a finite element based fully coupled algorithm that solved the flow/orientation coupling effects in extrusion deposition additive manufacturing, i.e., a key component for identifying the material properties of solidified deposited polymer composites. However, the complex iterative solution protocol reduces the computational capability of the algorithm. This paper simplifies the finite element based fully coupled algorithm by employing a scalar representation of the fourth order orientation tensor affected viscosity tensor. The revised code predicts similar fiber orientation results as compared to a related prior study. With the simplified fully coupled algorithm, effects of material deposition rate and fiber aspect ratio on a 2D planar large area extrusion deposition flow are explored, where the power law rheology model is employed for the flow simulation. Computed results show that an increased material deposition rate reduces the extrudate-swell ratio of the melt front and varies the local fiber orientation of the deposited bead especially near the flow region in contact with the substrate. Higher fiber aspect ratio enhances the principal fiber alignment of the extrusion and deposition flows, where the maximum local difference of the principal A 11 component is 23% with increasing fiber aspect ratio from 10 to 25.

The work explores the fundamentals of the softening process for amorphous materials expanding on the work presented by Stammers and Beek (Stammers, E. and Beek, W.J. (1969). The melting of a polymer on a hot surface. Polym. Eng. Sci. 9). The pressure induced melt removal process was studied in detail. The process, although fundamental, allows for an understanding of the phase transition of materials during the fused filament fabrication (FFF) process. Experimental results were analyzed using state-of-the-art equipment. A total of four materials were tested. PVC and PC are amorphous materials, and PA6 and PP are semi-crystalline materials. A setup was built to allow the application of a constant force. The resulting melt film was analyzed using computed tomography technology. The experimental results were compared with numerical simulation results originating from COMSOL Multiphysics and the analytical solution derived by Stammers and Beek. Rheological and thermo-mechanical tests were used to find the softening temperature of amorphous materials. Results showed a straightforward transition for the melting temperature of semi-crystalline materials and softening temperature of amorphous materials. It is suggested that for amorphous materials, the glass transition temperature can be used as the softening temperature ( T s  =  T g ). When comparing all three results (experimental, analytical, and numerical), values are within the same range and magnitude. A good approximation was achieved for PA6 and PVC, which have semi-crystalline and amorphous structures, respectively. Results provide insights into the phase transition process and phase change mechanisms during the extrusion of materials in the fused filament fabrication process.